diff --git a/src/mir/join_ir/lowering/if_select.rs b/src/mir/join_ir/lowering/if_select.rs
index 09e3129e..6fd477e6 100644
--- a/src/mir/join_ir/lowering/if_select.rs
+++ b/src/mir/join_ir/lowering/if_select.rs
@@ -3,44 +3,227 @@
 //! 最小の if/else（副作用なし、単純な値選択）を JoinInst::Select に変換する。
 
 use crate::mir::join_ir::JoinInst;
-use crate::mir::{BasicBlockId, MirFunction};
+use crate::mir::{BasicBlockId, MirFunction, MirInstruction, ValueId};
 
 pub struct IfSelectLowerer {
     debug: bool,
 }
 
+/// If/Else パターンの分類
+#[derive(Debug, Clone, Copy)]
+enum IfPatternType {
+    /// Simple pattern: if cond { return 1 } else { return 2 }
+    Simple,
+    /// Local pattern: if cond { x = a } else { x = b }; return x
+    Local,
+}
+
+/// 検出された If/Else パターン情報
+#[derive(Debug, Clone)]
+struct IfPattern {
+    pattern_type: IfPatternType,
+    cond: ValueId,
+    then_val: ValueId,
+    else_val: ValueId,
+    dst: Option<ValueId>,
+}
+
+/// Branch 命令の情報
+#[derive(Debug, Clone)]
+struct IfBranch {
+    cond: ValueId,
+    then_block: BasicBlockId,
+    else_block: BasicBlockId,
+}
+
 impl IfSelectLowerer {
     pub fn new(debug: bool) -> Self {
         Self { debug }
     }
 
     /// if/else が Select に lowering できるかチェック
-    pub fn can_lower_to_select(&self, _func: &MirFunction, _if_block_id: BasicBlockId) -> bool {
-        // パターン:
-        // 1. if_block に Branch がある
-        // 2. then/else ブロックが存在
-        // 3. merge ブロックに 1 つの PHI がある
-        // 4. PHI の incoming が then/else から来ている
-
-        // 実装: Phase 33-2 では保守的に false を返す（フォールバック）
-        // Phase 33-3 で実装予定
-        if self.debug {
-            eprintln!("[joinir/if_select] can_lower_to_select: Phase 33-2 stub (always false)");
-        }
-        false
+    pub fn can_lower_to_select(&self, func: &MirFunction, if_block_id: BasicBlockId) -> bool {
+        self.find_if_pattern(func, if_block_id).is_some()
     }
 
     /// if/else を Select に変換
     pub fn lower_if_to_select(
         &self,
-        _func: &MirFunction,
-        _if_block_id: BasicBlockId,
+        func: &MirFunction,
+        if_block_id: BasicBlockId,
     ) -> Option<JoinInst> {
-        // 実装: Phase 33-2 では None を返す（フォールバック）
-        // Phase 33-3 で実装予定
+        let pattern = self.find_if_pattern(func, if_block_id)?;
+
         if self.debug {
-            eprintln!("[joinir/if_select] lower_if_to_select: Phase 33-2 stub (always None)");
+            eprintln!(
+                "[IfSelectLowerer] lowering {:?} pattern to Select",
+                pattern.pattern_type
+            );
+        }
+
+        // Select 命令を生成
+        let dst = pattern.dst.unwrap_or(pattern.then_val);
+
+        Some(JoinInst::Select {
+            dst,
+            cond: pattern.cond,
+            then_val: pattern.then_val,
+            else_val: pattern.else_val,
+        })
+    }
+
+    /// MIR 関数から if/else パターンを探す
+    fn find_if_pattern(
+        &self,
+        func: &MirFunction,
+        block_id: BasicBlockId,
+    ) -> Option<IfPattern> {
+        // 1. Block が Branch 命令で終わっているか確認
+        let block = func.blocks.get(&block_id)?;
+        let branch = match block.terminator.as_ref()? {
+            MirInstruction::Branch {
+                condition,
+                then_bb,
+                else_bb,
+            } => IfBranch {
+                cond: *condition,
+                then_block: *then_bb,
+                else_block: *else_bb,
+            },
+            _ => return None,
+        };
+
+        // 2. then/else ブロックの構造を確認
+        let then_block = func.blocks.get(&branch.then_block)?;
+        let else_block = func.blocks.get(&branch.else_block)?;
+
+        // 3. simple パターンのチェック
+        if let Some(pattern) = self.try_match_simple_pattern(&branch, then_block, else_block) {
+            if self.debug {
+                eprintln!("[IfSelectLowerer] matched simple pattern");
+            }
+            return Some(pattern);
+        }
+
+        // 4. local パターンのチェック
+        if let Some(pattern) = self.try_match_local_pattern(func, &branch, then_block, else_block)
+        {
+            if self.debug {
+                eprintln!("[IfSelectLowerer] matched local pattern");
+            }
+            return Some(pattern);
+        }
+
+        if self.debug {
+            eprintln!("[IfSelectLowerer] no pattern matched");
         }
         None
     }
+
+    /// simple パターン: if cond { return 1 } else { return 2 }
+    fn try_match_simple_pattern(
+        &self,
+        branch: &IfBranch,
+        then_block: &crate::mir::BasicBlock,
+        else_block: &crate::mir::BasicBlock,
+    ) -> Option<IfPattern> {
+        // then ブロックが Return だけか確認
+        let then_val = match then_block.terminator.as_ref()? {
+            MirInstruction::Return { value: Some(v) } => *v,
+            _ => return None,
+        };
+
+        // else ブロックが Return だけか確認
+        let else_val = match else_block.terminator.as_ref()? {
+            MirInstruction::Return { value: Some(v) } => *v,
+            _ => return None,
+        };
+
+        // 両方のブロックが命令を持たない（Return のみ）ことを確認
+        if !then_block.instructions.is_empty() || !else_block.instructions.is_empty() {
+            return None;
+        }
+
+        Some(IfPattern {
+            pattern_type: IfPatternType::Simple,
+            cond: branch.cond,
+            then_val,
+            else_val,
+            dst: None,
+        })
+    }
+
+    /// local パターン: if cond { x = a } else { x = b }; return x
+    fn try_match_local_pattern(
+        &self,
+        func: &MirFunction,
+        branch: &IfBranch,
+        then_block: &crate::mir::BasicBlock,
+        else_block: &crate::mir::BasicBlock,
+    ) -> Option<IfPattern> {
+        // then ブロックが「1命令 + Jump」の形か確認
+        if then_block.instructions.len() != 1 {
+            return None;
+        }
+
+        // then ブロックの命令が代入（Copy）か確認
+        let (dst_then, val_then) = match &then_block.instructions[0] {
+            MirInstruction::Copy { dst, src } => (*dst, *src),
+            _ => return None,
+        };
+
+        // then ブロックが Jump で終わるか確認
+        let merge_block_id = match then_block.terminator.as_ref()? {
+            MirInstruction::Jump { target } => *target,
+            _ => return None,
+        };
+
+        // else ブロックも同じ構造か確認
+        if else_block.instructions.len() != 1 {
+            return None;
+        }
+
+        let (dst_else, val_else) = match &else_block.instructions[0] {
+            MirInstruction::Copy { dst, src } => (*dst, *src),
+            _ => return None,
+        };
+
+        // 代入先が同じ変数か確認
+        if dst_then != dst_else {
+            return None;
+        }
+
+        // else ブロックも同じ merge ブロックに Jump するか確認
+        let else_merge = match else_block.terminator.as_ref()? {
+            MirInstruction::Jump { target } => *target,
+            _ => return None,
+        };
+
+        if merge_block_id != else_merge {
+            return None;
+        }
+
+        // merge ブロックが「return dst」だけか確認
+        let merge_block = func.blocks.get(&merge_block_id)?;
+        match merge_block.terminator.as_ref()? {
+            MirInstruction::Return {
+                value: Some(v),
+            } if *v == dst_then => {
+                // OK
+            }
+            _ => return None,
+        }
+
+        if !merge_block.instructions.is_empty() {
+            return None;
+        }
+
+        Some(IfPattern {
+            pattern_type: IfPatternType::Local,
+            cond: branch.cond,
+            then_val: val_then,
+            else_val: val_else,
+            dst: Some(dst_then),
+        })
+    }
 }
diff --git a/src/mir/join_ir/lowering/mod.rs b/src/mir/join_ir/lowering/mod.rs
index f788f89f..3ce488e6 100644
--- a/src/mir/join_ir/lowering/mod.rs
+++ b/src/mir/join_ir/lowering/mod.rs
@@ -41,3 +41,61 @@ pub use skip_ws::lower_skip_ws_to_joinir;
 pub use stage1_using_resolver::lower_stage1_usingresolver_to_joinir;
 pub use stageb_body::lower_stageb_body_to_joinir;
 pub use stageb_funcscanner::lower_stageb_funcscanner_to_joinir;
+
+// Phase 33: If/Else → Select lowering entry point
+use crate::mir::join_ir::JoinInst;
+use crate::mir::{BasicBlockId, MirFunction};
+
+/// Phase 33-3: Try to lower if/else to JoinIR Select instruction
+///
+/// Scope:
+/// - Only applies to functions matching "IfSelectTest.*"
+/// - Requires NYASH_JOINIR_IF_SELECT=1 environment variable
+/// - Falls back to traditional if_phi on pattern mismatch
+///
+/// Returns Some(JoinInst::Select) if pattern matched, None otherwise.
+pub fn try_lower_if_to_joinir(
+    func: &MirFunction,
+    block_id: BasicBlockId,
+    debug: bool,
+) -> Option<JoinInst> {
+    // dev トグルチェック
+    if !crate::config::env::joinir_if_select_enabled() {
+        return None;
+    }
+
+    // 関数名で制限（IfSelectTest.* のみ）
+    if !func.signature.name.starts_with("IfSelectTest.") {
+        if debug {
+            eprintln!(
+                "[try_lower_if_to_joinir] skipping non-test function: {}",
+                func.signature.name
+            );
+        }
+        return None;
+    }
+
+    // if_select lowering を試行
+    let lowerer = if_select::IfSelectLowerer::new(debug);
+
+    if !lowerer.can_lower_to_select(func, block_id) {
+        if debug {
+            eprintln!(
+                "[try_lower_if_to_joinir] pattern not matched for {}",
+                func.signature.name
+            );
+        }
+        return None;
+    }
+
+    let result = lowerer.lower_if_to_select(func, block_id);
+
+    if result.is_some() && debug {
+        eprintln!(
+            "[try_lower_if_to_joinir] if_select lowering used for {}",
+            func.signature.name
+        );
+    }
+
+    result
+}
diff --git a/src/tests/mir_joinir_if_select.rs b/src/tests/mir_joinir_if_select.rs
new file mode 100644
index 00000000..c3a22c89
--- /dev/null
+++ b/src/tests/mir_joinir_if_select.rs
@@ -0,0 +1,232 @@
+//! Phase 33-3: If/Else → Select lowering integration tests
+//!
+//! Tests the pattern matching and lowering of if/else to JoinIR Select instruction.
+
+#[cfg(test)]
+mod tests {
+    use crate::mir::join_ir::lowering::try_lower_if_to_joinir;
+    use crate::mir::join_ir::JoinInst;
+    use crate::mir::{
+        BasicBlock, BasicBlockId, MirFunction, MirInstruction, MirModule, ValueId,
+    };
+    use std::collections::BTreeMap;
+
+    /// Helper to create a simple if/else function matching the "simple" pattern
+    fn create_simple_pattern_mir() -> MirFunction {
+        let mut blocks = BTreeMap::new();
+
+        // Entry block (bb0): branch on cond
+        let mut entry = BasicBlock::new(BasicBlockId::new(0));
+        entry.terminator = Some(MirInstruction::Branch {
+            condition: ValueId(0), // cond parameter
+            then_bb: BasicBlockId::new(1),
+            else_bb: BasicBlockId::new(2),
+        });
+        blocks.insert(BasicBlockId::new(0), entry);
+
+        // Then block (bb1): return 10
+        // NOTE: Pattern matcher expects empty blocks (Return only)
+        let mut then_block = BasicBlock::new(BasicBlockId::new(1));
+        then_block.terminator = Some(MirInstruction::Return {
+            value: Some(ValueId(1)), // Assumes ValueId(1) is const 10
+        });
+        blocks.insert(BasicBlockId::new(1), then_block);
+
+        // Else block (bb2): return 20
+        // NOTE: Pattern matcher expects empty blocks (Return only)
+        let mut else_block = BasicBlock::new(BasicBlockId::new(2));
+        else_block.terminator = Some(MirInstruction::Return {
+            value: Some(ValueId(2)), // Assumes ValueId(2) is const 20
+        });
+        blocks.insert(BasicBlockId::new(2), else_block);
+
+        use crate::mir::{EffectMask, MirType};
+        use crate::mir::function::FunctionMetadata;
+        use std::collections::HashMap;
+
+        MirFunction {
+            signature: crate::mir::FunctionSignature {
+                name: "IfSelectTest.test/1".to_string(),
+                params: vec![MirType::Unknown],
+                return_type: MirType::Integer,
+                effects: EffectMask::PURE,
+            },
+            entry_block: BasicBlockId::new(0),
+            blocks: blocks.into_iter().collect(),
+            locals: vec![],
+            params: vec![ValueId(0)],
+            next_value_id: 3,
+            metadata: FunctionMetadata::default(),
+        }
+    }
+
+    /// Helper to create a local pattern function
+    fn create_local_pattern_mir() -> MirFunction {
+        let mut blocks = BTreeMap::new();
+
+        // Entry block (bb0): branch on cond
+        let mut entry = BasicBlock::new(BasicBlockId::new(0));
+        entry.terminator = Some(MirInstruction::Branch {
+            condition: ValueId(0), // cond
+            then_bb: BasicBlockId::new(1),
+            else_bb: BasicBlockId::new(2),
+        });
+        blocks.insert(BasicBlockId::new(0), entry);
+
+        // Then block (bb1): x = 100; jump merge
+        // NOTE: Pattern matcher expects exactly 1 Copy instruction
+        let mut then_block = BasicBlock::new(BasicBlockId::new(1));
+        then_block.instructions.push(MirInstruction::Copy {
+            dst: ValueId(3), // x
+            src: ValueId(10), // Assumes ValueId(10) is const 100
+        });
+        then_block.terminator = Some(MirInstruction::Jump {
+            target: BasicBlockId::new(3),
+        });
+        blocks.insert(BasicBlockId::new(1), then_block);
+
+        // Else block (bb2): x = 200; jump merge
+        // NOTE: Pattern matcher expects exactly 1 Copy instruction
+        let mut else_block = BasicBlock::new(BasicBlockId::new(2));
+        else_block.instructions.push(MirInstruction::Copy {
+            dst: ValueId(3), // x
+            src: ValueId(20), // Assumes ValueId(20) is const 200
+        });
+        else_block.terminator = Some(MirInstruction::Jump {
+            target: BasicBlockId::new(3),
+        });
+        blocks.insert(BasicBlockId::new(2), else_block);
+
+        // Merge block (bb3): return x
+        let mut merge_block = BasicBlock::new(BasicBlockId::new(3));
+        merge_block.terminator = Some(MirInstruction::Return {
+            value: Some(ValueId(3)),
+        });
+        blocks.insert(BasicBlockId::new(3), merge_block);
+
+        use crate::mir::{EffectMask, MirType};
+        use crate::mir::function::FunctionMetadata;
+        use std::collections::HashMap;
+
+        MirFunction {
+            signature: crate::mir::FunctionSignature {
+                name: "IfSelectTest.main/0".to_string(),
+                params: vec![],
+                return_type: MirType::Integer,
+                effects: EffectMask::PURE,
+            },
+            entry_block: BasicBlockId::new(0),
+            blocks: blocks.into_iter().collect(),
+            locals: vec![],
+            params: vec![],
+            next_value_id: 21,
+            metadata: FunctionMetadata::default(),
+        }
+    }
+
+    #[test]
+    fn test_if_select_simple_pattern() {
+        // Set environment variable for this test
+        std::env::set_var("NYASH_JOINIR_IF_SELECT", "1");
+
+        let func = create_simple_pattern_mir();
+        let entry_block = func.entry_block;
+
+        // Try to lower to JoinIR
+        let result = try_lower_if_to_joinir(&func, entry_block, true);
+
+        assert!(
+            result.is_some(),
+            "Expected simple pattern to be lowered to Select"
+        );
+
+        if let Some(JoinInst::Select {
+            dst,
+            cond,
+            then_val,
+            else_val,
+        }) = result
+        {
+            eprintln!("✅ Simple pattern successfully lowered to Select");
+            eprintln!("   dst: {:?}, cond: {:?}, then: {:?}, else: {:?}", dst, cond, then_val, else_val);
+        } else {
+            panic!("Expected JoinInst::Select, got {:?}", result);
+        }
+
+        // Clean up
+        std::env::remove_var("NYASH_JOINIR_IF_SELECT");
+    }
+
+    #[test]
+    fn test_if_select_local_pattern() {
+        std::env::set_var("NYASH_JOINIR_IF_SELECT", "1");
+
+        let func = create_local_pattern_mir();
+        let entry_block = func.entry_block;
+
+        // Try to lower to JoinIR
+        let result = try_lower_if_to_joinir(&func, entry_block, true);
+
+        assert!(
+            result.is_some(),
+            "Expected local pattern to be lowered to Select"
+        );
+
+        if let Some(JoinInst::Select {
+            dst,
+            cond,
+            then_val,
+            else_val,
+        }) = result
+        {
+            eprintln!("✅ Local pattern successfully lowered to Select");
+            eprintln!("   dst: {:?}, cond: {:?}, then: {:?}, else: {:?}", dst, cond, then_val, else_val);
+        } else {
+            panic!("Expected JoinInst::Select, got {:?}", result);
+        }
+
+        std::env::remove_var("NYASH_JOINIR_IF_SELECT");
+    }
+
+    #[test]
+    fn test_if_select_disabled_by_default() {
+        // Ensure environment variable is NOT set
+        std::env::remove_var("NYASH_JOINIR_IF_SELECT");
+
+        let func = create_simple_pattern_mir();
+        let entry_block = func.entry_block;
+
+        // Should return None when disabled
+        let result = try_lower_if_to_joinir(&func, entry_block, false);
+
+        assert!(
+            result.is_none(),
+            "Expected None when NYASH_JOINIR_IF_SELECT is not set"
+        );
+
+        eprintln!("✅ If/Select lowering correctly disabled by default");
+    }
+
+    #[test]
+    fn test_if_select_wrong_function_name() {
+        std::env::set_var("NYASH_JOINIR_IF_SELECT", "1");
+
+        // Create function with wrong name (not IfSelectTest.*)
+        let mut func = create_simple_pattern_mir();
+        func.signature.name = "WrongName.test/1".to_string();
+
+        let entry_block = func.entry_block;
+
+        // Should return None for non-IfSelectTest functions
+        let result = try_lower_if_to_joinir(&func, entry_block, true);
+
+        assert!(
+            result.is_none(),
+            "Expected None for non-IfSelectTest functions"
+        );
+
+        eprintln!("✅ Function name filter working correctly");
+
+        std::env::remove_var("NYASH_JOINIR_IF_SELECT");
+    }
+}
diff --git a/src/tests/mod.rs b/src/tests/mod.rs
index 8aad7e37..de0d2ebf 100644
--- a/src/tests/mod.rs
+++ b/src/tests/mod.rs
@@ -20,6 +20,7 @@ pub mod mir_funcscanner_ssa;
 pub mod mir_funcscanner_trim_min;
 pub mod mir_joinir_funcscanner_append_defs; // Phase 27.14: FuncScannerBox._append_defs JoinIR変換
 pub mod mir_joinir_funcscanner_trim; // Phase 27.1: FuncScannerBox.trim JoinIR変換
+pub mod mir_joinir_if_select; // Phase 33-3: If/Else → Select lowering tests
 pub mod mir_joinir_min; // Phase 26-H: JoinIR型定義妥当性確認
 pub mod mir_joinir_skip_ws; // Phase 27.0: minimal_ssa_skip_ws JoinIR変換
 pub mod mir_joinir_stage1_using_resolver_min; // Phase 27.12: Stage1UsingResolverBox.resolve_for_source JoinIR変換