diff --git a/CHANGES.rst b/CHANGES.rst
index a383f3f4..930fd7e8 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -20,6 +20,8 @@ Unreleased
     (`#3059`_)
 -   :func:`send_file` supports :class:`~io.BytesIO` partial content.
     (`#2957`_)
+-   :func:`open_resource` accepts the "rt" file mode. This still does
+    the same thing as "r". (:issue:`3163`)
 
 .. _#2935: https://github.com/pallets/flask/issues/2935
 .. _#2957: https://github.com/pallets/flask/issues/2957
diff --git a/flask/helpers.py b/flask/helpers.py
index 78f4b37c..c5e85a02 100644
--- a/flask/helpers.py
+++ b/flask/helpers.py
@@ -1053,12 +1053,12 @@ class _PackageBoundObject(object):
 
         :param resource: the name of the resource.  To access resources within
                          subfolders use forward slashes as separator.
-        :param mode: resource file opening mode, default is 'rb'.
+        :param mode: Open file in this mode. Only reading is supported,
+            valid values are "r" (or "rt") and "rb".
         """
-        if mode == 'rt':
-            mode = 'r'
-        if mode not in ("r", "rb"):
+        if mode not in {"r", "rt", "rb"}:
             raise ValueError("Resources can only be opened for reading")
+
         return open(os.path.join(self.root_path, resource), mode)
 
 
diff --git a/tests/test_helpers.py b/tests/test_helpers.py
index e7730da0..cddc7517 100644
--- a/tests/test_helpers.py
+++ b/tests/test_helpers.py
@@ -1062,26 +1062,16 @@ class TestHelpers(object):
             assert rv.data == b"Hello"
             assert rv.mimetype == "text/html"
 
-    @pytest.mark.parametrize('mode', [
-        'r',
-        'rb',
-        'rt'
-    ])
+    @pytest.mark.parametrize("mode", ("r", "rb", "rt"))
     def test_open_resource(self, mode):
         app = flask.Flask(__name__)
-        with app.open_resource('static/index.html', mode) as f:
-            assert '<h1>Hello World!</h1>' in str(f.read())
 
-    @pytest.mark.parametrize('mode', [
-        'w',
-        'x',
-        'a',
-        'b',
-        't',
-        '+'
-    ])
+        with app.open_resource("static/index.html", mode) as f:
+            assert "<h1>Hello World!</h1>" in str(f.read())
+
+    @pytest.mark.parametrize("mode", ("w", "x", "a", "r+"))
     def test_open_resource_exceptions(self, mode):
         app = flask.Flask(__name__)
-        with pytest.raises(ValueError):
-            app.open_resource('static/index.html', mode)
 
+        with pytest.raises(ValueError):
+            app.open_resource("static/index.html", mode)