diff --git a/scrapy/spidermiddlewares/offsite.py b/scrapy/spidermiddlewares/offsite.py
index ea1c9270f..310166cad 100644
--- a/scrapy/spidermiddlewares/offsite.py
+++ b/scrapy/spidermiddlewares/offsite.py
@@ -6,6 +6,7 @@ See documentation in docs/topics/spider-middleware.rst
 
 import re
 import logging
+import warnings
 
 from scrapy import signals
 from scrapy.http import Request
@@ -52,9 +53,18 @@ class OffsiteMiddleware(object):
         allowed_domains = getattr(spider, 'allowed_domains', None)
         if not allowed_domains:
             return re.compile('') # allow all by default
+        url_pattern = re.compile("^https?://.*$")
+        for domain in allowed_domains:
+            if url_pattern.match(domain):
+                warnings.warn("allowed_domains accepts only domains, not URLs. Ignoring URL entry %s in allowed_domains." % domain, URLWarning)
+                
         regex = r'^(.*\.)?(%s)$' % '|'.join(re.escape(d) for d in allowed_domains if d is not None)
         return re.compile(regex)
 
     def spider_opened(self, spider):
         self.host_regex = self.get_host_regex(spider)
         self.domains_seen = set()
+
+
+class URLWarning(Warning):
+    pass
diff --git a/tests/test_spidermiddleware_offsite.py b/tests/test_spidermiddleware_offsite.py
index 9ad86313c..7e4af0d4c 100644
--- a/tests/test_spidermiddleware_offsite.py
+++ b/tests/test_spidermiddleware_offsite.py
@@ -5,7 +5,9 @@ from six.moves.urllib.parse import urlparse
 from scrapy.http import Response, Request
 from scrapy.spiders import Spider
 from scrapy.spidermiddlewares.offsite import OffsiteMiddleware
+from scrapy.spidermiddlewares.offsite import URLWarning
 from scrapy.utils.test import get_crawler
+import warnings
 
 class TestOffsiteMiddleware(TestCase):
 
@@ -68,3 +70,13 @@ class TestOffsiteMiddleware4(TestOffsiteMiddleware3):
       reqs = [Request('http://scrapytest.org/1')]
       out = list(self.mw.process_spider_output(res, reqs, self.spider))
       self.assertEqual(out, reqs)
+
+
+class TestOffsiteMiddleware5(TestOffsiteMiddleware4):
+    
+    def test_get_host_regex(self):
+        self.spider.allowed_domains = ['http://scrapytest.org', 'scrapy.org', 'scrapy.test.org']
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
+            self.mw.get_host_regex(self.spider)
+            assert issubclass(w[-1].category, URLWarning)