diff --git a/lib/document.js b/lib/document.js
index 3e0e42cc1ef..a8614bbb749 100644
--- a/lib/document.js
+++ b/lib/document.js
@@ -31,6 +31,7 @@ const inspect = require('util').inspect;
 const internalToObjectOptions = require('./options').internalToObjectOptions;
 const mpath = require('mpath');
 const utils = require('./utils');
+const isPromise = require('./helpers/isPromise');
 
 const clone = utils.clone;
 const deepEqual = utils.deepEqual;
@@ -3075,7 +3076,7 @@ Document.prototype.$toObject = function(options, json) {
   // we need to adjust options.transform to be the child schema's transform and
   // not the parent schema's
   if (transform) {
-    applySchemaTypeTransforms(this, ret, gettersOptions, options);
+    applySchemaTypeTransforms(this, ret);
   }
 
   if (transform === true || (schemaOptions.toObject && transform)) {
@@ -3414,13 +3415,17 @@ function applySchemaTypeTransforms(self, json) {
     const schematype = schema.paths[path];
     if (typeof schematype.options.transform === 'function') {
       const val = self.get(path);
-      json[path] = schematype.options.transform.call(self, val);
+      const transformedValue = schematype.options.transform.call(self, val);
+      throwErrorIfPromise(path, transformedValue);
+      json[path] = transformedValue;
     } else if (schematype.$embeddedSchemaType != null &&
         typeof schematype.$embeddedSchemaType.options.transform === 'function') {
       const vals = [].concat(self.get(path));
       const transform = schematype.$embeddedSchemaType.options.transform;
       for (let i = 0; i < vals.length; ++i) {
-        vals[i] = transform.call(self, vals[i]);
+        const transformedValue = transform.call(self, vals[i]);
+        vals[i] = transformedValue;
+        throwErrorIfPromise(path, transformedValue);
       }
 
       json[path] = vals;
@@ -3430,6 +3435,12 @@ function applySchemaTypeTransforms(self, json) {
   return json;
 }
 
+function throwErrorIfPromise(path, transformedValue) {
+  if (isPromise(transformedValue)) {
+    throw new Error('`transform` function must be synchronous, but the transform on path `' + path + '` returned a promise.');
+  }
+}
+
 /**
  * The return value of this method is used in calls to JSON.stringify(doc).
  *
diff --git a/lib/helpers/isPromise.js b/lib/helpers/isPromise.js
new file mode 100644
index 00000000000..d6db2608cc9
--- /dev/null
+++ b/lib/helpers/isPromise.js
@@ -0,0 +1,6 @@
+'use strict';
+function isPromise(val) {
+  return !!val && (typeof val === 'object' || typeof val === 'function') && typeof val.then === 'function';
+}
+
+module.exports = isPromise;
\ No newline at end of file
diff --git a/test/document.test.js b/test/document.test.js
index 8020a362b36..9ef1d2d3e00 100644
--- a/test/document.test.js
+++ b/test/document.test.js
@@ -8976,6 +8976,28 @@ describe('document', function() {
     assert.equal(axl.fullName, 'Axl Rose');
   });
 
+  it('throws an error when `transform` returns a promise (gh-9163)', function() {
+    const userSchema = new Schema({
+      name: {
+        type: String,
+        transform: function() {
+          return new Promise(() => {});
+        }
+      }
+    });
+
+    const User = db.model('User', userSchema);
+
+    const user = new User({ name: 'Hafez' });
+    assert.throws(function() {
+      user.toJSON();
+    }, /`transform` option has to be synchronous, but is returning a promise on path `name`./);
+
+    assert.throws(function() {
+      user.toObject();
+    }, /`transform` option has to be synchronous, but is returning a promise on path `name`./);
+  });
+    
   it('uses strict equality when checking mixed paths for modifications (gh-9165)', function() {
     const schema = Schema({ obj: {} });
     const Model = db.model('gh9165', schema);