diff --git a/AUTHORS.rst b/AUTHORS.rst
index e684d850..37b66698 100644
--- a/AUTHORS.rst
+++ b/AUTHORS.rst
@@ -165,3 +165,4 @@ Patches and Suggestions
 - Brian Samek (`@bsamek <https://github.com/bsamek>`_)
 - Dmitry Dygalo (`@Stranger6667 <https://github.com/Stranger6667>`_)
 - piotrjurkiewicz
+- Jesse Shapiro <jesse@jesseshapiro.net> (`@haikuginger <https://github.com/haikuginger>`_)
diff --git a/requests/models.py b/requests/models.py
index 0c10eef1..67747405 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -420,8 +420,12 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
         length = None
 
         if not data and json is not None:
+            # urllib3 requires a bytes-like body. Python 2's json.dumps
+            # provides this natively, but Python 3 gives a Unicode string.
             content_type = 'application/json'
             body = complexjson.dumps(json)
+            if not isinstance(body, bytes):
+                body = body.encode('utf-8')
 
         is_stream = all([
             hasattr(data, '__iter__'),
diff --git a/tests/test_requests.py b/tests/test_requests.py
index 4bcf4536..167d5ca7 100755
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -1528,6 +1528,18 @@ class RedirectSession(SessionRedirectMixin):
         return string
 
 
+def test_json_encodes_as_bytes():
+    # urllib3 expects bodies as bytes-like objects
+    body = {"key": "value"}
+    p = PreparedRequest()
+    p.prepare(
+        method='GET',
+        url='https://www.example.com/',
+        json=body
+    )
+    assert isinstance(p.body, bytes)
+
+
 def test_requests_are_updated_each_time(httpbin):
     session = RedirectSession([303, 307])
     prep = requests.Request('POST', httpbin('post')).prepare()