diff --git a/src/requests/compat.py b/src/requests/compat.py
index 095de1b6ca..3725bbb62a 100644
--- a/src/requests/compat.py
+++ b/src/requests/compat.py
@@ -10,6 +10,18 @@
 import importlib
 import sys
 
+# -------
+# urllib3
+# -------
+from urllib3 import __version__ as urllib3_version
+
+# Detect which major version of urllib3 is being used.
+try:
+    is_urllib3_2 = int(urllib3_version.split('.')[0]) == 2
+except (TypeError, AttributeError):
+    # If we can't discern a version, prefer old functionality.
+    is_urllib3_2 = False
+
 # -------------------
 # Character Detection
 # -------------------
diff --git a/src/requests/utils.py b/src/requests/utils.py
index ae6c42f6cb..5cbd8d6e6e 100644
--- a/src/requests/utils.py
+++ b/src/requests/utils.py
@@ -41,6 +41,7 @@
 )
 from .compat import parse_http_list as _parse_list_header
 from .compat import (
+    is_urllib3_2,
     proxy_bypass,
     proxy_bypass_environment,
     quote,
@@ -136,7 +137,9 @@ def super_len(o):
     total_length = None
     current_position = 0
 
-    if isinstance(o, str):
+    if is_urllib3_2 and isinstance(o, str):
+        # urllib3 2.x treats all strings as utf-8 instead
+        # of latin-1 (iso-8859-1) like http.client.
         o = o.encode("utf-8")
 
     if hasattr(o, "__len__"):