diff --git a/rust/cpp.rs b/rust/cpp.rs
index e79c9ba902..77e0454553 100644
--- a/rust/cpp.rs
+++ b/rust/cpp.rs
@@ -415,7 +415,7 @@ macro_rules! impl_ProxiedInMapValue_for_non_generated_value_types {
                 fn [< __rust_proto_thunk__Map_ $key_t _ $t _free >](m: RawMap);
                 fn [< __rust_proto_thunk__Map_ $key_t _ $t _clear >](m: RawMap);
                 fn [< __rust_proto_thunk__Map_ $key_t _ $t _size >](m: RawMap) -> usize;
-                fn [< __rust_proto_thunk__Map_ $key_t _ $t _insert >](m: RawMap, key: $ffi_key_t, value: $ffi_t);
+                fn [< __rust_proto_thunk__Map_ $key_t _ $t _insert >](m: RawMap, key: $ffi_key_t, value: $ffi_t) -> bool;
                 fn [< __rust_proto_thunk__Map_ $key_t _ $t _get >](m: RawMap, key: $ffi_key_t, value: *mut $ffi_t) -> bool;
                 fn [< __rust_proto_thunk__Map_ $key_t _ $t _remove >](m: RawMap, key: $ffi_key_t, value: *mut $ffi_t) -> bool;
             }
@@ -453,7 +453,6 @@ macro_rules! impl_ProxiedInMapValue_for_non_generated_value_types {
                     let ffi_key = $to_ffi_key(key);
                     let ffi_value = $to_ffi_value(value);
                     unsafe { [< __rust_proto_thunk__Map_ $key_t _ $t _insert >](map.inner.raw, ffi_key, ffi_value) }
-                    true
                 }
 
                 fn map_get<'a>(map: View<'a, Map<$key_t, Self>>, key: View<'_, $key_t>) -> Option<View<'a, Self>> {
diff --git a/rust/cpp_kernel/cpp_api.cc b/rust/cpp_kernel/cpp_api.cc
index 404f6f60a6..0666308182 100644
--- a/rust/cpp_kernel/cpp_api.cc
+++ b/rust/cpp_kernel/cpp_api.cc
@@ -115,11 +115,10 @@ expose_repeated_ptr_field_methods(Bytes);
       const google::protobuf::Map<key_ty, value_ty>* m) {                                \
     return m->size();                                                          \
   }                                                                            \
-  void __rust_proto_thunk__Map_##rust_key_ty##_##rust_value_ty##_insert(       \
+  bool __rust_proto_thunk__Map_##rust_key_ty##_##rust_value_ty##_insert(       \
       google::protobuf::Map<key_ty, value_ty>* m, ffi_key_ty key, ffi_value_ty value) {  \
-    auto cpp_key = to_cpp_key;                                                 \
-    auto cpp_value = to_cpp_value;                                             \
-    (*m)[cpp_key] = cpp_value;                                                 \
+    auto iter_and_inserted = m->try_emplace(to_cpp_key, to_cpp_value);         \
+    return iter_and_inserted.second;                                           \
   }                                                                            \
   bool __rust_proto_thunk__Map_##rust_key_ty##_##rust_value_ty##_get(          \
       const google::protobuf::Map<key_ty, value_ty>* m, ffi_key_ty key,                  \
diff --git a/rust/map.rs b/rust/map.rs
index 43f59c3c05..d7368f80ca 100644
--- a/rust/map.rs
+++ b/rust/map.rs
@@ -231,6 +231,9 @@ where
         self.len() == 0
     }
 
+    /// Adds a key-value pair to the map.
+    ///
+    /// Returns `true` if the entry was newly inserted.
     pub fn insert<'a, 'b>(
         &mut self,
         key: impl Into<View<'a, K>>,
diff --git a/rust/test/shared/accessors_map_test.rs b/rust/test/shared/accessors_map_test.rs
index 19c020ad3a..85f9fb5c4c 100644
--- a/rust/test/shared/accessors_map_test.rs
+++ b/rust/test/shared/accessors_map_test.rs
@@ -20,6 +20,7 @@ macro_rules! generate_map_primitives_tests {
                 let k: $k_type = Default::default();
                 let v: $v_type = Default::default();
                 assert_that!(msg.[< map_ $k_field _ $v_field _mut>]().insert(k, v), eq(true));
+                assert_that!(msg.[< map_ $k_field _ $v_field _mut>]().insert(k, v), eq(false));
                 assert_that!(msg.[< map_ $k_field _ $v_field >]().len(), eq(1));
             }
         )* }
diff --git a/rust/upb.rs b/rust/upb.rs
index 6a1c65e169..b28bc01a0e 100644
--- a/rust/upb.rs
+++ b/rust/upb.rs
@@ -832,7 +832,7 @@ macro_rules! impl_ProxiedInMapValue_for_non_generated_value_types {
 
                 fn map_insert(map: Mut<'_, Map<$key_t, Self>>, key: View<'_, $key_t>, value: View<'_, Self>) -> bool {
                     unsafe {
-                        upb_Map_Set(
+                        upb_Map_InsertAndReturnIfInserted(
                             map.inner.raw,
                             <$key_t as UpbTypeConversions>::to_message_value(key),
                             <$t as UpbTypeConversions>::to_message_value_copy_if_required(map.inner.raw_arena, value),
@@ -878,15 +878,49 @@ macro_rules! impl_ProxiedInMapValue_for_key_types {
 
 impl_ProxiedInMapValue_for_key_types!(i32, u32, i64, u64, bool, ProtoStr);
 
+#[repr(C)]
+#[allow(dead_code)]
+enum upb_MapInsertStatus {
+    Inserted = 0,
+    Replaced = 1,
+    OutOfMemory = 2,
+}
+
+/// `upb_Map_Insert`, but returns a `bool` for whether insert occurred.
+///
+/// Returns `true` if the entry was newly inserted.
+///
+/// # Panics
+/// Panics if the arena is out of memory.
+///
+/// # Safety
+/// The same as `upb_Map_Insert`:
+/// - `map` must be a valid map.
+/// - The `arena` must be valid and outlive the map.
+/// - The inserted value must outlive the map.
+#[allow(non_snake_case)]
+pub unsafe fn upb_Map_InsertAndReturnIfInserted(
+    map: RawMap,
+    key: upb_MessageValue,
+    value: upb_MessageValue,
+    arena: RawArena,
+) -> bool {
+    match unsafe { upb_Map_Insert(map, key, value, arena) } {
+        upb_MapInsertStatus::Inserted => true,
+        upb_MapInsertStatus::Replaced => false,
+        upb_MapInsertStatus::OutOfMemory => panic!("map arena is out of memory"),
+    }
+}
+
 extern "C" {
     fn upb_Map_New(arena: RawArena, key_type: UpbCType, value_type: UpbCType) -> RawMap;
     fn upb_Map_Size(map: RawMap) -> usize;
-    fn upb_Map_Set(
+    fn upb_Map_Insert(
         map: RawMap,
         key: upb_MessageValue,
         value: upb_MessageValue,
         arena: RawArena,
-    ) -> bool;
+    ) -> upb_MapInsertStatus;
     fn upb_Map_Get(map: RawMap, key: upb_MessageValue, value: *mut upb_MessageValue) -> bool;
     fn upb_Map_Delete(
         map: RawMap,