diff --git a/src/ast/SideEffects.zig b/src/ast/SideEffects.zig
index 87152bebbc..7e1f318a41 100644
--- a/src/ast/SideEffects.zig
+++ b/src/ast/SideEffects.zig
@@ -790,6 +790,19 @@ pub const SideEffects = enum(u1) {
                                             }
                                         }
                                     }
+                                    // Check if the define value is undefined
+                                    if (define.value == .e_undefined) {
+                                        // window is undefined, so typeof window is "undefined"
+                                        if (e_.right.data == .e_string) {
+                                            const str = e_.right.data.e_string;
+                                            if (str.eqlComptime("undefined")) {
+                                                // typeof window === "undefined" -> true
+                                                // typeof window !== "undefined" -> false
+                                                const is_equal = e_.op == .bin_strict_eq or e_.op == .bin_loose_eq;
+                                                return Result{ .ok = true, .value = is_equal, .side_effects = .could_have_side_effects };
+                                            }
+                                        }
+                                    }
                                 }
                             }
                             // Handle typeof dot expression (e.g., typeof globalThis.Bun)
@@ -837,6 +850,19 @@ pub const SideEffects = enum(u1) {
                                             }
                                         }
                                     }
+                                    // Check if the define value is undefined
+                                    if (define.value == .e_undefined) {
+                                        // window is undefined, so typeof window is "undefined"
+                                        if (e_.left.data == .e_string) {
+                                            const str = e_.left.data.e_string;
+                                            if (str.eqlComptime("undefined")) {
+                                                // "undefined" === typeof window -> true
+                                                // "undefined" !== typeof window -> false
+                                                const is_equal = e_.op == .bin_strict_eq or e_.op == .bin_loose_eq;
+                                                return Result{ .ok = true, .value = is_equal, .side_effects = .could_have_side_effects };
+                                            }
+                                        }
+                                    }
                                 }
                             }
                             // Handle typeof dot expression (e.g., typeof globalThis.Bun)
diff --git a/src/options.zig b/src/options.zig
index e8e55dbd40..0c62f37aae 100644
--- a/src/options.zig
+++ b/src/options.zig
@@ -1511,6 +1511,11 @@ pub fn definesFromTransformOptions(
             }));
         }
         
+        // process.isBun - simple boolean true for Bun runtime detection
+        if (!user_defines.contains("process.isBun")) {
+            _ = try user_defines.getOrPutValue("process.isBun", "true");
+        }
+        
         // For --target=bun, mark these as truthy for DCE without replacing values
         // This enables dead code elimination while preserving runtime values
         
@@ -1545,6 +1550,17 @@ pub fn definesFromTransformOptions(
             }));
         }
     }
+    
+    // For --target=node, mark window as undefined for DCE
+    if (target.isNode()) {
+        if (!user_defines.contains("window")) {
+            _ = try environment_defines.getOrPutValue("window", .init(.{
+                .valueless = true,
+                .original_name = "window",
+                .value = .{ .e_undefined = .{} },
+            }));
+        }
+    }
 
     const resolved_defines = try defines.DefineData.fromInput(user_defines, drop, log, allocator);
 
diff --git a/test/bundler/bun-target-dce.test.ts b/test/bundler/bun-target-dce.test.ts
index 3a8f6f8fc5..7f230d451d 100644
--- a/test/bundler/bun-target-dce.test.ts
+++ b/test/bundler/bun-target-dce.test.ts
@@ -26,6 +26,13 @@ if (process.versions.bun) {
   require("./should-not-import-3.js");
 }
 
+// process.isBun - should be replaced with true
+if (process.isBun) {
+  exports.test3a = "process-isBun-true";
+} else {
+  exports.test3a = "process-isBun-false";
+}
+
 // ============ typeof checks ============
 if (typeof Bun !== "undefined") {
   exports.test4 = "typeof-bun-defined";
@@ -140,6 +147,7 @@ var require_HASH = __commonJS((exports) => {
   Bun, exports.test1 = "bun-exists";
   globalThis.Bun, exports.test2 = "globalThis-bun-exists";
   process.versions.bun, exports.test3 = "process-versions-bun-exists";
+  exports.test3a = "process-isBun-true";
   exports.test4 = "typeof-bun-defined";
   globalThis.Bun, exports.test5 = "typeof-globalThis-bun-defined";
   exports.test6 = "typeof-bun-reverse-defined";
@@ -171,10 +179,7 @@ var require_HASH = __commonJS((exports) => {
     exports.test17 = "node-version-exists";
   else
     exports.test17 = "node-version-missing";
-  if (typeof window !== "undefined")
-    exports.test18 = "window-exists";
-  else
-    exports.test18 = "window-missing";
+  exports.test18 = "window-missing";
   var isBun = typeof Bun !== "undefined";
   if (!isBun)
     exports.test19 = "const-not-bun";
@@ -210,7 +215,8 @@ export default require_HASH();"
   
   // Non-Bun runtime checks preserved
   expect(bundled).toContain("process.versions.node");
-  expect(bundled).toContain("typeof window");
+  // typeof window check is eliminated since window is undefined for bun target
+  expect(bundled).not.toContain("typeof window");
   
   // Const patterns don't work (needs constant propagation)
   expect(bundled).toContain("const-not-bun");
@@ -232,6 +238,20 @@ if (typeof Bun !== "undefined") {
 } else {
   exports.hasBun = false;
 }
+
+// window check - should be undefined for both bun and node targets
+if (typeof window === "undefined") {
+  exports.isServer = true;
+} else {
+  exports.isServer = false;
+}
+
+// process.isBun check
+if (process.isBun) {
+  exports.isBun = true;
+} else {
+  exports.isBun = false;
+}
   `;
 
   // Build for Bun
@@ -262,6 +282,14 @@ if (typeof Bun !== "undefined") {
   expect(bunBundle).not.toContain('exports.runtime = "unknown"');
   expect(bunBundle).toContain('exports.hasBun = !0'); // minified true
   expect(bunBundle).not.toContain('exports.hasBun = !1'); // minified false
+  
+  // window is undefined for bun target (server environment)
+  expect(bunBundle).toContain('exports.isServer = !0'); // true - window is undefined
+  expect(bunBundle).not.toContain('exports.isServer = !1'); // false branch eliminated
+  
+  // process.isBun is replaced with true for bun target
+  expect(bunBundle).toContain('exports.isBun = !0'); // true
+  expect(bunBundle).not.toContain('exports.isBun = !1'); // false branch eliminated
 
   // Node bundle should keep all branches (Bun is unknown at runtime)
   expect(nodeBundle).toContain('exports.runtime = "bun"');
@@ -269,4 +297,11 @@ if (typeof Bun !== "undefined") {
   expect(nodeBundle).toContain('exports.runtime = "unknown"');
   expect(nodeBundle).toContain('exports.hasBun = !0'); // minified true
   expect(nodeBundle).toContain('exports.hasBun = !1'); // minified false
+  
+  // window is undefined for node target (server environment)
+  expect(nodeBundle).toContain('exports.isServer = !0'); // true - window is undefined
+  expect(nodeBundle).not.toContain('exports.isServer = !1'); // false branch eliminated
+  
+  // process.isBun doesn't exist for node target - both branches kept
+  expect(nodeBundle).toContain('process.isBun'); // The check is still there
 });
\ No newline at end of file