diff --git a/openedx/core/djangoapps/util/tests/test_user_messages.py b/openedx/core/djangoapps/util/tests/test_user_messages.py
index 1cf74b305e..80758ee43d 100644
--- a/openedx/core/djangoapps/util/tests/test_user_messages.py
+++ b/openedx/core/djangoapps/util/tests/test_user_messages.py
@@ -3,6 +3,8 @@ Unit tests for user messages.
 """
 
 
+import warnings
+
 import ddt
 from django.contrib.messages.middleware import MessageMiddleware
 from django.test import RequestFactory, TestCase
@@ -76,3 +78,59 @@ class UserMessagesTestCase(TestCase):
         messages = list(PageLevelMessages.user_messages(self.request))
         assert len(messages) == 1
         assert messages[0].type == expected_message_type
+
+    def global_message_count(self):
+        """
+        Count the number of times the global message appears in the user messages.
+        """
+        expected_html = """<div class="message-content">I &lt;3 HTML-escaping</div>"""
+        messages = list(PageLevelMessages.user_messages(self.request))
+        return len(list(msg for msg in messages if expected_html in msg.message_html))
+
+    def test_global_message_off_by_default(self):
+        """Verifies feature toggle."""
+        with self.settings(
+            GLOBAL_NOTICE_ENABLED=False,
+            GLOBAL_NOTICE_MESSAGE="I <3 HTML-escaping",
+            GLOBAL_NOTICE_TYPE='WARNING'
+        ):
+            # Missing when feature disabled
+            assert self.global_message_count() == 0
+
+    def test_global_message_persistent(self):
+        """Verifies global message is always included, when enabled."""
+        with self.settings(
+            GLOBAL_NOTICE_ENABLED=True,
+            GLOBAL_NOTICE_MESSAGE="I <3 HTML-escaping",
+            GLOBAL_NOTICE_TYPE='WARNING'
+        ):
+            # Present with no other setup
+            assert self.global_message_count() == 1
+
+            # Present when other messages are present
+            PageLevelMessages.register_user_message(self.request, UserMessageType.INFO, "something else")
+            assert self.global_message_count() == 1
+
+    def test_global_message_error_isolation(self):
+        """Verifies that any setting errors don't break the page, or other messages."""
+        with self.settings(
+            GLOBAL_NOTICE_ENABLED=True,
+            GLOBAL_NOTICE_MESSAGE=ThrowingMarkup(),  # force an error
+            GLOBAL_NOTICE_TYPE='invalid'
+        ):
+            PageLevelMessages.register_user_message(self.request, UserMessageType.WARNING, "something else")
+            # Doesn't throw, or even interfere with other messages,
+            # when given invalid settings
+            with warnings.catch_warnings(record=True) as w:
+                warnings.simplefilter('always')
+                messages = list(PageLevelMessages.user_messages(self.request))
+                assert len(w) == 1
+                assert str(w[0].message) == "Could not register global notice: Exception('Some random error')"
+            assert len(messages) == 1
+            assert "something else" in messages[0].message_html
+
+
+class ThrowingMarkup:
+    """Class that raises an exception if markupsafe tries to get HTML from it."""
+    def __html__(self):
+        raise Exception("Some random error")
diff --git a/openedx/core/djangoapps/util/user_messages.py b/openedx/core/djangoapps/util/user_messages.py
index 1a5c6ed6cb..e6438c58a3 100644
--- a/openedx/core/djangoapps/util/user_messages.py
+++ b/openedx/core/djangoapps/util/user_messages.py
@@ -15,11 +15,14 @@ There are two common use cases:
 """
 
 
+import warnings
 from abc import abstractmethod
 from enum import Enum
 
+from django.conf import settings
 from django.contrib import messages
 from django.utils.translation import ugettext as _
+from edx_toggles.toggles import SettingToggle
 
 from openedx.core.djangolib.markup import HTML, Text
 
@@ -181,12 +184,54 @@ class UserMessageCollection():
         return (_create_user_message(message) for message in django_messages if self.get_namespace() in message.tags)
 
 
+# .. toggle_name: GLOBAL_NOTICE_ENABLED
+# .. toggle_implementation: SettingToggle
+# .. toggle_default: False
+# .. toggle_description: When enabled, show the contents of GLOBAL_NOTICE_MESSAGE
+#   as a message on every page. This is intended to be used as a way of
+#   communicating an upcoming or currently active maintenance window or to
+#   warn of known site issues. HTML is not supported for the message content,
+#   only plaintext. Message styling can be controlled with GLOBAL_NOTICE_TYPE,
+#   set to one of INFO, SUCCESS, WARNING, or ERROR (defaulting to INFO). Also
+#   see openedx.core.djangoapps.util.maintenance_banner.add_maintenance_banner
+#   for a variation that only shows a message on specific views.
+# .. toggle_use_cases: open_edx
+# .. toggle_creation_date: 2021-09-08
+GLOBAL_NOTICE_ENABLED = SettingToggle('GLOBAL_NOTICE_ENABLED', default=False)
+
+
 class PageLevelMessages(UserMessageCollection):
     """
     This set of messages appears as top page level messages.
     """
     NAMESPACE = 'page_level_messages'
 
+    @classmethod
+    def user_messages(cls, request):
+        """
+        Returns outstanding user messages, along with any persistent site-wide messages.
+        """
+        msgs = list(super().user_messages(request))
+
+        # Add a global notice message to the returned list, if enabled.
+        try:
+            if GLOBAL_NOTICE_ENABLED.is_enabled():
+                if notice_message := getattr(settings, 'GLOBAL_NOTICE_MESSAGE', None):
+                    notice_type_str = getattr(settings, 'GLOBAL_NOTICE_TYPE', None)
+                    # If an invalid type is given, better to show a
+                    # message with the default type than to fail to
+                    # show it at all.
+                    notice_type = getattr(UserMessageType, notice_type_str, UserMessageType.INFO)
+
+                msgs.append(UserMessage(
+                    type=notice_type,
+                    message_html=str(cls.get_message_html(Text(notice_message))),
+                ))
+        except BaseException as e:
+            warnings.warn(f"Could not register global notice: {e!r}", UserWarning)
+
+        return msgs
+
     @classmethod
     def get_message_html(cls, body_html, title=None, dismissable=False, **kwargs):
         """