Index: Objects/stringobject.c
===================================================================
--- Objects/stringobject.c	(revision 66118)
+++ Objects/stringobject.c	(working copy)
@@ -1638,7 +1638,7 @@
 	}
 #ifdef Py_USING_UNICODE
 	else if (PyUnicode_Check(sep_obj))
-		return PyUnicode_Partition((PyObject *) self, sep_obj);
+		return PyUnicode_RPartition((PyObject *) self, sep_obj);
 #endif
 	else if (PyObject_AsCharBuffer(sep_obj, &sep, &sep_len))
 		return NULL;
Index: Lib/test/string_tests.py
===================================================================
--- Lib/test/string_tests.py	(revision 66118)
+++ Lib/test/string_tests.py	(working copy)
@@ -1117,6 +1117,9 @@
         self.checkraises(ValueError, S, 'partition', '')
         self.checkraises(TypeError, S, 'partition', None)
 
+        # mixed use of str and unicode
+        self.assertEqual('a/b/c'.partition(u'/'), ('a', '/', 'b/c'))
+
     def test_rpartition(self):
 
         self.checkequal(('this is the rparti', 'ti', 'on method'),
@@ -1132,6 +1135,8 @@
         self.checkraises(ValueError, S, 'rpartition', '')
         self.checkraises(TypeError, S, 'rpartition', None)
 
+        # mixed use of str and unicode
+        self.assertEqual('a/b/c'.rpartition(u'/'), ('a/b', '/', 'c'))
 
 class MixinStrStringUserStringTest:
     # Additional tests for 8bit strings, i.e. str, UserString and
Index: Misc/NEWS
===================================================================
--- Misc/NEWS	(revision 66118)
+++ Misc/NEWS	(working copy)
@@ -12,6 +12,9 @@
 Core and Builtins
 -----------------
 
+- Issue #3751: str.rpartition would perform a left-partition when called with
+  a unicode argument.
+
 - Issue #3683: Fix compilation when --without-threads is given.
 
 - Issue #3668: Fix a memory leak with the "s*" argument parser in