diff --git a/stdlib/3/datetime.pyi b/stdlib/3/datetime.pyi
index 527867da8..8b1b2e45a 100644
--- a/stdlib/3/datetime.pyi
+++ b/stdlib/3/datetime.pyi
@@ -1,3 +1,4 @@
+import sys
 from typing import Optional, SupportsAbs, Tuple, overload
 
 MINYEAR = 0
@@ -198,7 +199,10 @@ class datetime:
                 Optional[_tzinfo] = None) -> datetime: ...
     def astimezone(self, tz: Optional[_tzinfo] = ...) -> datetime: ...
     def ctime(self) -> str: ...
-    def isoformat(self, sep: str = ...) -> str: ...
+    if sys.version_info >= (3, 6):
+        def isoformat(self, sep: str = ..., timespec: str = ...) -> str: ...
+    else:
+        def isoformat(self, sep: str = ...) -> str: ...
     @classmethod
     def strptime(cls, date_string: str, format: str) -> datetime: ...
     def utcoffset(self) -> Optional[timedelta]: ...