diff --git a/pymongo/message.py b/pymongo/message.py
index 19361cbf2..23da7deb6 100644
--- a/pymongo/message.py
+++ b/pymongo/message.py
@@ -73,13 +73,16 @@ def _maybe_add_read_preference(spec, read_preference):
     """Add $readPreference to spec when appropriate."""
     mode = read_preference.mode
     tag_sets = read_preference.tag_sets
+    max_staleness = read_preference.max_staleness
     # Only add $readPreference if it's something other than primary to avoid
     # problems with mongos versions that don't support read preferences. Also,
     # for maximum backwards compatibility, don't add $readPreference for
-    # secondaryPreferred unless tags are in use (setting the slaveOkay bit
-    # has the same effect).
+    # secondaryPreferred unless tags or maxStalenessMS are in use (setting the
+    # slaveOkay bit has the same effect).
     if mode and (
-        mode != ReadPreference.SECONDARY_PREFERRED.mode or tag_sets != [{}]):
+        mode != ReadPreference.SECONDARY_PREFERRED.mode
+        or tag_sets != [{}]
+        or max_staleness):
 
         if "$query" not in spec:
             spec = SON([("$query", spec)])
diff --git a/pymongo/read_preferences.py b/pymongo/read_preferences.py
index 1be0df014..19cfc2432 100644
--- a/pymongo/read_preferences.py
+++ b/pymongo/read_preferences.py
@@ -63,11 +63,11 @@ def _validate_tag_sets(tag_sets):
 
 
 def _validate_max_staleness(max_staleness):
-    """Validate maxStalenessMS."""
+    """Validate max_staleness."""
     if max_staleness is None:
         return 0.0
 
-    errmsg = "maxStalenessMS must be an integer or float"
+    errmsg = "max_staleness must be an integer or float"
     try:
         max_staleness = float(max_staleness)
     except ValueError:
@@ -75,6 +75,10 @@ def _validate_max_staleness(max_staleness):
     except TypeError:
         raise TypeError(errmsg)
 
+    if not 0 < max_staleness < 1e9:
+        raise ValueError(
+            "max_staleness must be greater than 0 and less than one billion")
+
     return max_staleness
 
 
@@ -100,9 +104,12 @@ class _ServerMode(object):
     def document(self):
         """Read preference as a document.
         """
-        if self.__tag_sets in (None, [{}]):
-            return {'mode': self.__mongos_mode}
-        return {'mode': self.__mongos_mode, 'tags': self.__tag_sets}
+        doc = {'mode': self.__mongos_mode}
+        if self.__tag_sets not in (None, [{}]):
+            doc['tags'] = self.__tag_sets
+        if self.__max_staleness:
+            doc['maxStalenessMS'] = int(self.__max_staleness * 1000)
+        return doc
 
     @property
     def mode(self):
@@ -130,6 +137,8 @@ class _ServerMode(object):
         """The maximum estimated length of time (in seconds) a replica set
         secondary can fall behind the primary in replication before it will
         no longer be selected for operations."""
+        if not self.__max_staleness:
+            return None
         return self.__max_staleness
 
     @property
@@ -146,8 +155,8 @@ class _ServerMode(object):
         return 5 if self.__max_staleness else 0
 
     def __repr__(self):
-        return "%s(tag_sets=%r)" % (
-            self.name, self.__tag_sets)
+        return "%s(tag_sets=%r, max_staleness=%r)" % (
+            self.name, self.__tag_sets, self.max_staleness)
 
     def __eq__(self, other):
         if isinstance(other, _ServerMode):
diff --git a/test/test_max_staleness.py b/test/test_max_staleness.py
index 2e307829e..c6c9f351e 100644
--- a/test/test_max_staleness.py
+++ b/test/test_max_staleness.py
@@ -159,6 +159,8 @@ def create_test(scenario_def):
         mode_string = mode_string[:1].lower() + mode_string[1:]
         mode = read_preferences.read_pref_mode_from_name(mode_string)
         max_staleness = pref_def.get('maxStalenessMS', 0) / 1000.0
+        if not max_staleness:
+            max_staleness = None
         tag_sets = pref_def.get('tag_sets')
 
         if scenario_def.get('error'):
diff --git a/test/test_read_preferences.py b/test/test_read_preferences.py
index a8488eb6f..18fcb6dbc 100644
--- a/test/test_read_preferences.py
+++ b/test/test_read_preferences.py
@@ -24,7 +24,7 @@ sys.path[0:0] = [""]
 
 from bson.py3compat import MAXSIZE
 from bson.son import SON
-from pymongo.errors import ConfigurationError
+from pymongo.errors import ConfigurationError, OperationFailure
 from pymongo.message import _maybe_add_read_preference
 from pymongo.mongo_client import MongoClient
 from pymongo.read_preferences import (ReadPreference, MovingAverage,
@@ -446,6 +446,77 @@ class TestMovingAverage(unittest.TestCase):
 
 class TestMongosAndReadPreference(unittest.TestCase):
 
+    def test_read_preference_document(self):
+
+        pref = Primary()
+        self.assertEqual(
+            pref.document,
+            {'mode': 'primary'})
+
+        pref = PrimaryPreferred()
+        self.assertEqual(
+            pref.document,
+            {'mode': 'primaryPreferred'})
+        pref = PrimaryPreferred(tag_sets=[{'dc': 'sf'}])
+        self.assertEqual(
+            pref.document,
+            {'mode': 'primaryPreferred', 'tags': [{'dc': 'sf'}]})
+        pref = PrimaryPreferred(
+            tag_sets=[{'dc': 'sf'}], max_staleness=30)
+        self.assertEqual(
+            pref.document,
+            {'mode': 'primaryPreferred',
+             'tags': [{'dc': 'sf'}],
+             'maxStalenessMS': 30000})
+
+        pref = Secondary()
+        self.assertEqual(
+            pref.document,
+            {'mode': 'secondary'})
+        pref = Secondary(tag_sets=[{'dc': 'sf'}])
+        self.assertEqual(
+            pref.document,
+            {'mode': 'secondary', 'tags': [{'dc': 'sf'}]})
+        pref = Secondary(
+            tag_sets=[{'dc': 'sf'}], max_staleness=30)
+        self.assertEqual(
+            pref.document,
+            {'mode': 'secondary',
+             'tags': [{'dc': 'sf'}],
+             'maxStalenessMS': 30000})
+
+        pref = SecondaryPreferred()
+        self.assertEqual(
+            pref.document,
+            {'mode': 'secondaryPreferred'})
+        pref = SecondaryPreferred(tag_sets=[{'dc': 'sf'}])
+        self.assertEqual(
+            pref.document,
+            {'mode': 'secondaryPreferred', 'tags': [{'dc': 'sf'}]})
+        pref = SecondaryPreferred(
+            tag_sets=[{'dc': 'sf'}], max_staleness=30)
+        self.assertEqual(
+            pref.document,
+            {'mode': 'secondaryPreferred',
+             'tags': [{'dc': 'sf'}],
+             'maxStalenessMS': 30000})
+
+        pref = Nearest()
+        self.assertEqual(
+            pref.document,
+            {'mode': 'nearest'})
+        pref = Nearest(tag_sets=[{'dc': 'sf'}])
+        self.assertEqual(
+            pref.document,
+            {'mode': 'nearest', 'tags': [{'dc': 'sf'}]})
+        pref = Nearest(
+            tag_sets=[{'dc': 'sf'}], max_staleness=30)
+        self.assertEqual(
+            pref.document,
+            {'mode': 'nearest',
+             'tags': [{'dc': 'sf'}],
+             'maxStalenessMS': 30000})
+
     def test_maybe_add_read_preference(self):
 
         # Primary doesn't add $readPreference
@@ -470,12 +541,17 @@ class TestMongosAndReadPreference(unittest.TestCase):
         self.assertEqual(
             out, SON([("$query", {}), ("$readPreference", pref.document)]))
 
-        # SecondaryPreferred without tag_sets doesn't add $readPreference
+        # SecondaryPreferred without tag_sets or max_staleness doesn't add
+        # $readPreference
         pref = SecondaryPreferred()
         out = _maybe_add_read_preference({}, pref)
         self.assertEqual(out, {})
         pref = SecondaryPreferred(tag_sets=[{'dc': 'nyc'}])
         out = _maybe_add_read_preference({}, pref)
+        self.assertEqual(
+            out, SON([("$query", {}), ("$readPreference", pref.document)]))
+        pref = SecondaryPreferred(max_staleness=120)
+        out = _maybe_add_read_preference({}, pref)
         self.assertEqual(
             out, SON([("$query", {}), ("$readPreference", pref.document)]))
 
@@ -533,6 +609,39 @@ class TestMongosAndReadPreference(unittest.TestCase):
             self.assertEqual(first_id, results[-1]["_id"])
             self.assertEqual(last_id, results[0]["_id"])
 
+    @client_context.require_mongos
+    @client_context.require_version_min(3, 3, 12)
+    def test_mongos_max_staleness(self):
+        # Sanity check that we're sending maxStalenessMS
+        coll = client_context.client.pymongo_test.get_collection(
+            "test", read_preference=SecondaryPreferred(max_staleness=120))
+        # No error
+        coll.find_one()
+
+        coll = client_context.client.pymongo_test.get_collection(
+            "test", read_preference=SecondaryPreferred(max_staleness=10))
+        try:
+            coll.find_one()
+        except OperationFailure as exc:
+            self.assertEqual(160, exc.code)
+        else:
+            self.fail("mongos accepted invalid staleness")
+
+        coll = single_client(
+            readPreference='secondaryPreferred',
+            maxStalenessMS=120000).pymongo_test.test
+        # No error
+        coll.find_one()
+
+        coll = single_client(
+            readPreference='secondaryPreferred',
+            maxStalenessMS=10000).pymongo_test.test
+        try:
+            coll.find_one()
+        except OperationFailure as exc:
+            self.assertEqual(160, exc.code)
+        else:
+            self.fail("mongos accepted invalid staleness")
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/test/test_topology.py b/test/test_topology.py
index 3d3066d0f..f1e0eb22b 100644
--- a/test/test_topology.py
+++ b/test/test_topology.py
@@ -710,12 +710,12 @@ class TestServerSelectionErrors(TopologyTest):
 
         self.assertMessage(
             'No replica set members match selector'
-            ' "Secondary(tag_sets=None)"',
+            ' "Secondary(tag_sets=None, max_staleness=None)"',
             t, ReadPreference.SECONDARY)
 
         self.assertMessage(
             "No replica set members match selector"
-            " \"Secondary(tag_sets=[{'dc': 'ny'}])\"",
+            " \"Secondary(tag_sets=[{'dc': 'ny'}], max_staleness=None)\"",
             t, Secondary(tag_sets=[{'dc': 'ny'}]))
 
     def test_bad_replica_set_name(self):