diff --git a/CHANGES b/CHANGES
index 207da344..cc763166 100644
--- a/CHANGES
+++ b/CHANGES
@@ -97,6 +97,8 @@ Major release, unreleased
     - ``Request.module`` - use ``Request.blueprint`` instead.
 
 - The ``request.json`` property is no longer deprecated. (`#1421`_)
+- Support passing an existing ``EnvironBuilder`` or ``dict`` to
+  ``test_client.open``. (`#2412`_)
 
 .. _#1421: https://github.com/pallets/flask/issues/1421
 .. _#1489: https://github.com/pallets/flask/pull/1489
@@ -124,6 +126,7 @@ Major release, unreleased
 .. _#2374: https://github.com/pallets/flask/pull/2374
 .. _#2373: https://github.com/pallets/flask/pull/2373
 .. _#2385: https://github.com/pallets/flask/issues/2385
+.. _#2412: https://github.com/pallets/flask/pull/2412
 
 Version 0.12.2
 --------------
diff --git a/flask/testing.py b/flask/testing.py
index f73454af..d5109ce1 100644
--- a/flask/testing.py
+++ b/flask/testing.py
@@ -144,19 +144,41 @@ class FlaskClient(Client):
             self.cookie_jar.extract_wsgi(c.request.environ, headers)
 
     def open(self, *args, **kwargs):
-        kwargs.setdefault('environ_overrides', {}) \
-            ['flask._preserve_context'] = self.preserve_context
-        kwargs.setdefault('environ_base', self.environ_base)
-
         as_tuple = kwargs.pop('as_tuple', False)
         buffered = kwargs.pop('buffered', False)
         follow_redirects = kwargs.pop('follow_redirects', False)
-        builder = make_test_environ_builder(self.application, *args, **kwargs)
 
-        return Client.open(self, builder,
-                           as_tuple=as_tuple,
-                           buffered=buffered,
-                           follow_redirects=follow_redirects)
+        if (
+            not kwargs and len(args) == 1
+            and isinstance(args[0], (EnvironBuilder, dict))
+        ):
+            environ = self.environ_base.copy()
+
+            if isinstance(args[0], EnvironBuilder):
+                environ.update(args[0].get_environ())
+            else:
+                environ.update(args[0])
+
+            environ['flask._preserve_context'] = self.preserve_context
+        else:
+            kwargs.setdefault('environ_overrides', {}) \
+                ['flask._preserve_context'] = self.preserve_context
+            kwargs.setdefault('environ_base', self.environ_base)
+            builder = make_test_environ_builder(
+                self.application, *args, **kwargs
+            )
+
+            try:
+                environ = builder.get_environ()
+            finally:
+                builder.close()
+
+        return Client.open(
+            self, environ,
+            as_tuple=as_tuple,
+            buffered=buffered,
+            follow_redirects=follow_redirects
+        )
 
     def __enter__(self):
         if self.preserve_context:
diff --git a/tests/test_testing.py b/tests/test_testing.py
index 068eb209..e94da483 100644
--- a/tests/test_testing.py
+++ b/tests/test_testing.py
@@ -15,6 +15,7 @@ import werkzeug
 
 from flask._compat import text_type
 from flask.json import jsonify
+from flask.testing import make_test_environ_builder
 
 
 def test_environ_defaults_from_config(app, client):
@@ -74,6 +75,23 @@ def test_environ_base_modified(app, client, app_ctx):
     assert flask.g.user_agent == 'Bar'
 
 
+def test_client_open_environ(app, client, request):
+    @app.route('/index')
+    def index():
+        return flask.request.remote_addr
+
+    builder = make_test_environ_builder(app, path='/index', method='GET')
+    request.addfinalizer(builder.close)
+
+    rv = client.open(builder)
+    assert rv.data == b'127.0.0.1'
+
+    environ = builder.get_environ()
+    client.environ_base['REMOTE_ADDR'] = '127.0.0.2'
+    rv = client.open(environ)
+    assert rv.data == b'127.0.0.2'
+
+
 def test_specify_url_scheme(app, client):
     @app.route('/')
     def index():