diff --git a/src/cli/args.rs b/src/cli/args.rs
index 907ff02e..791d6a2f 100644
--- a/src/cli/args.rs
+++ b/src/cli/args.rs
@@ -105,6 +105,12 @@ pub fn build_command() -> Command {
         .arg(Arg::new("build-aot").long("build-aot").value_name("{cranelift|llvm}").help("AOT backend for --build"))
         .arg(Arg::new("build-profile").long("profile").value_name("{release|debug}").help("Cargo profile for --build"))
         .arg(Arg::new("build-target").long("target").value_name("TRIPLE").help("Target triple for --build"))
+        // Phase 288 P1: REPL mode
+        .arg(Arg::new("repl")
+            .long("repl")
+            .short('i')
+            .help("Start interactive REPL (Read-Eval-Print Loop)")
+            .action(clap::ArgAction::SetTrue))
 }
 
 fn hex_encode_utf8(s: &str) -> String {
@@ -206,6 +212,8 @@ pub fn from_matches(matches: &ArgMatches) -> CliConfig {
         macro_expand_child: matches.get_one::<String>("macro-expand-child").cloned(),
         dump_expanded_ast_json: matches.get_flag("dump-expanded-ast-json"),
         macro_ctx_json: matches.get_one::<String>("macro-ctx-json").cloned(),
+        // Phase 288 P1: REPL mode
+        repl: matches.get_flag("repl"),
     };
 
     if cfg.cli_verbose {
diff --git a/src/cli/mod.rs b/src/cli/mod.rs
index 2ce1955a..13684d06 100644
--- a/src/cli/mod.rs
+++ b/src/cli/mod.rs
@@ -70,6 +70,8 @@ pub struct CliConfig {
     pub macro_expand_child: Option<String>,
     pub dump_expanded_ast_json: bool,
     pub macro_ctx_json: Option<String>,
+    // Phase 288 P1: REPL mode
+    pub repl: bool,
 }
 
 pub use groups::{
@@ -221,6 +223,8 @@ impl Default for CliConfig {
             macro_expand_child: None,
             dump_expanded_ast_json: false,
             macro_ctx_json: None,
+            // Phase 288 P1: REPL mode
+            repl: false,
         }
     }
 }
diff --git a/src/runner/mod.rs b/src/runner/mod.rs
index 3b2ca951..b2c79a9e 100644
--- a/src/runner/mod.rs
+++ b/src/runner/mod.rs
@@ -88,6 +88,70 @@ impl NyashRunner {
     fn run_build_mvp(&self, cfg_path: &str) -> Result<(), String> {
         build::run_build_mvp_impl(self, cfg_path)
     }
+
+    /// Phase 288 P1: Run REPL mode
+    fn run_repl(&self) -> ! {
+        use std::io::{self, Write};
+
+        println!("Nyash REPL v1.0 - Phase 288 MVP");
+        println!("Type .help for commands, .exit to quit");
+
+        let stdin = io::stdin();
+        let mut line_buf = String::new();
+
+        loop {
+            print!(">>> ");
+            io::stdout().flush().unwrap();
+
+            line_buf.clear();
+            match stdin.read_line(&mut line_buf) {
+                Ok(0) => break,  // EOF
+                Ok(_) => {
+                    let line = line_buf.trim();
+
+                    // REPL commands
+                    match line {
+                        ".exit" | ".quit" => break,
+                        ".help" => {
+                            println!("Commands:");
+                            println!("  .exit / .quit - Exit REPL");
+                            println!("  .reset - Clear session (Phase 288 P2)");
+                            println!("  .help - Show this help");
+                            continue;
+                        }
+                        ".reset" => {
+                            println!("Session reset (Phase 288 P2 implementation)");
+                            continue;
+                        }
+                        "" => continue,
+                        _ => {}
+                    }
+
+                    // Evaluate line
+                    match self.eval_repl_line(line) {
+                        Ok(result) => {
+                            if !result.is_empty() {
+                                println!("{}", result);
+                            }
+                        }
+                        Err(e) => eprintln!("Error: {}", e),
+                    }
+                }
+                Err(e) => {
+                    eprintln!("Input error: {}", e);
+                    break;
+                }
+            }
+        }
+
+        println!("Goodbye!");
+        std::process::exit(0);
+    }
+
+    /// Phase 288 P1: Evaluate REPL line (stub)
+    fn eval_repl_line(&self, _line: &str) -> Result<String, String> {
+        Ok("(evaluation Phase 288 P2)".to_string())
+    }
 }
 
 #[cfg(not(feature = "jit-direct-only"))]
@@ -99,6 +163,10 @@ impl NyashRunner {
             crate::runner::modes::macro_child::run_macro_child(macro_file);
             return;
         }
+        // Phase 288 P1: REPL mode
+        if self.config.repl {
+            self.run_repl();  // never returns
+        }
         let groups = self.config.as_groups();
 
         // CLI mode trace: show backend/file/args when emit-mir trace is enabled