diff --git a/scrapy/core/spidermw.py b/scrapy/core/spidermw.py
index 5a99b96be..dd0cf675f 100644
--- a/scrapy/core/spidermw.py
+++ b/scrapy/core/spidermw.py
@@ -10,7 +10,7 @@ from twisted.python.failure import Failure
 from scrapy.exceptions import _InvalidOutput
 from scrapy.middleware import MiddlewareManager
 from scrapy.utils.conf import build_component_list
-from scrapy.utils.defer import mustbe_deferred
+from scrapy.utils.defer import mustbe_deferred, deferred_f_from_coro_f
 from scrapy.utils.python import MutableChain
 
 
@@ -38,7 +38,7 @@ class SpiderMiddlewareManager(MiddlewareManager):
         if hasattr(mw, 'process_spider_input'):
             self.methods['process_spider_input'].append(mw.process_spider_input)
         if hasattr(mw, 'process_start_requests'):
-            self.methods['process_start_requests'].appendleft(mw.process_start_requests)
+            self.methods['process_start_requests'].appendleft(deferred_f_from_coro_f(mw.process_start_requests))
         process_spider_output = getattr(mw, 'process_spider_output', None)
         self.methods['process_spider_output'].appendleft(process_spider_output)
         process_spider_exception = getattr(mw, 'process_spider_exception', None)
diff --git a/tests/test_spidermiddleware.py b/tests/test_spidermiddleware.py
index 7adf9a84b..6f931e8f8 100644
--- a/tests/test_spidermiddleware.py
+++ b/tests/test_spidermiddleware.py
@@ -1,3 +1,4 @@
+import collections
 import inspect
 import sys
 from unittest import mock
@@ -115,6 +116,11 @@ class ProcessStartRequestsSimpleMiddleware:
             yield r
 
 
+class ProcessStartRequestsAsyncDefMiddleware:
+    async def process_start_requests(self, start_requests, spider):
+        return start_requests
+
+
 class ProcessStartRequestsSimple(TestCase):
     """ process_start_requests tests for simple start_requests"""
 
@@ -139,7 +145,7 @@ class ProcessStartRequestsSimple(TestCase):
     @defer.inlineCallbacks
     def _test_simple_base(self, *mw_classes):
         processed_start_requests = yield self._get_processed_start_requests(*mw_classes)
-        self.assertTrue(inspect.isgenerator(processed_start_requests))
+        self.assertIsInstance(processed_start_requests, collections.abc.Iterable)
         start_requests_list = list(processed_start_requests)
         self.assertEqual(len(start_requests_list), 3)
         self.assertIsInstance(start_requests_list[0], Request)
@@ -158,6 +164,11 @@ class ProcessStartRequestsSimple(TestCase):
         """ Simple mw """
         yield self._test_simple_base(ProcessStartRequestsSimpleMiddleware)
 
+    @defer.inlineCallbacks
+    def test_asyncdef(self):
+        """ Async def mw """
+        yield self._test_simple_base(ProcessStartRequestsAsyncDefMiddleware)
+
     @mark.skipif(sys.version_info < (3, 6), reason="Async generators require Python 3.6 or higher")
     @defer.inlineCallbacks
     def test_asyncgen(self):
@@ -208,6 +219,11 @@ class ProcessStartRequestsAsyncGen(ProcessStartRequestsSimple):
         self.assertTrue(inspect.isgenerator(processed_start_requests))
         self.assertAsyncGeneratorNotIterable(processed_start_requests)
 
+    @defer.inlineCallbacks
+    def test_asyncdef(self):
+        """ Async def mw """
+        yield self._test_asyncgen_base(ProcessStartRequestsAsyncDefMiddleware)
+
     @defer.inlineCallbacks
     def test_simple_asyncgen(self):
         """ Simple mw -> asyncgen mw; cannot work """