diff --git a/src/parser/serializer.ts b/src/parser/serializer.ts
index baf3fe9b..076bd3eb 100644
--- a/src/parser/serializer.ts
+++ b/src/parser/serializer.ts
@@ -757,7 +757,11 @@ export function serializeInto(
 
       // Get the entry values
       const key = entry.value[0];
-      const value = entry.value[1];
+      let value = entry.value[1];
+
+      if (typeof value?.toBSON === 'function') {
+        value = value.toBSON();
+      }
 
       // Check the type of the value
       const type = typeof value;
diff --git a/test/node/to_bson_test.js b/test/node/to_bson_test.js
index 1178cda3..0c78ab90 100644
--- a/test/node/to_bson_test.js
+++ b/test/node/to_bson_test.js
@@ -183,6 +183,15 @@ describe('toBSON', function () {
       const sizeNestedToBSON = BSON.calculateObjectSize({ a: [0] });
       expect(sizeNestedToBSON).to.equal(33);
     });
+
+    it('uses toBSON on values contained in a map', () => {
+      const map = new Map();
+      map.set('a', 100);
+
+      const serializedData = BSON.serialize(map);
+      const deserializedData = BSON.deserialize(serializedData);
+      expect(deserializedData).to.have.property('a', 'hello number');
+    });
   });
 
   it('should use toBSON in calculateObjectSize', () => {