diff --git a/lms/djangoapps/courseware/access.py b/lms/djangoapps/courseware/access.py
index b03144888f83..46668f1aa864 100644
--- a/lms/djangoapps/courseware/access.py
+++ b/lms/djangoapps/courseware/access.py
@@ -27,6 +27,7 @@
     MilestoneAccessError,
     MobileAvailabilityError,
     NoAllowedPartitionGroupsError,
+    OldMongoAccessError,
     VisibilityError
 )
 from lms.djangoapps.courseware.access_utils import (
@@ -329,6 +330,9 @@ def can_load():
         # ).or(
         #     _has_staff_access_to_descriptor, user, courselike, courselike.id
         # )
+        if courselike.id.deprecated:  # we no longer support accessing Old Mongo courses
+            return OldMongoAccessError(courselike)
+
         visible_to_nonstaff = _visible_to_nonstaff_users(courselike)
         if not visible_to_nonstaff:
             staff_access = _has_staff_access_to_descriptor(user, courselike, courselike.id)
diff --git a/lms/djangoapps/courseware/access_response.py b/lms/djangoapps/courseware/access_response.py
index e88b58d2051d..674003d6c85f 100644
--- a/lms/djangoapps/courseware/access_response.py
+++ b/lms/djangoapps/courseware/access_response.py
@@ -236,3 +236,16 @@ def __init__(self):
         developer_message = "User must be authenticated to view the course"
         user_message = _("You must be logged in to see this course")
         super().__init__(error_code, developer_message, user_message)
+
+
+class OldMongoAccessError(AccessError):
+    """
+    Access denied because the course is in Old Mongo and we no longer support them. See DEPR-58.
+    """
+    def __init__(self, courselike):
+        error_code = 'old_mongo'
+        developer_message = 'Access to Old Mongo courses is unsupported'
+        user_message = _('{course_name} is no longer available.').format(
+            course_name=courselike.display_name_with_default,
+        )
+        super().__init__(error_code, developer_message, user_message)
diff --git a/lms/djangoapps/courseware/courses.py b/lms/djangoapps/courseware/courses.py
index 89fa7c0596fa..cd755a8d476f 100644
--- a/lms/djangoapps/courseware/courses.py
+++ b/lms/djangoapps/courseware/courses.py
@@ -27,6 +27,7 @@
     AuthenticationRequiredAccessError,
     EnrollmentRequiredAccessError,
     MilestoneAccessError,
+    OldMongoAccessError,
     StartDateError,
 )
 from lms.djangoapps.courseware.date_summary import (
@@ -213,6 +214,15 @@ def check_course_access_with_redirect(course, user, action, check_if_enrolled=Fa
                 params=params.urlencode()
             ), access_response)
 
+        # Redirect if trying to access an Old Mongo course
+        if isinstance(access_response, OldMongoAccessError):
+            params = QueryDict(mutable=True)
+            params['access_response_error'] = access_response.user_message
+            raise CourseAccessRedirect('{dashboard_url}?{params}'.format(
+                dashboard_url=reverse('dashboard'),
+                params=params.urlencode(),
+            ), access_response)
+
         # Redirect if the user must answer a survey before entering the course.
         if isinstance(access_response, MilestoneAccessError):
             raise CourseAccessRedirect('{dashboard_url}'.format(
diff --git a/lms/djangoapps/courseware/tests/test_courses.py b/lms/djangoapps/courseware/tests/test_courses.py
index 365dc0aaf734..c81686507046 100644
--- a/lms/djangoapps/courseware/tests/test_courses.py
+++ b/lms/djangoapps/courseware/tests/test_courses.py
@@ -39,6 +39,7 @@
     get_courses,
     get_current_child
 )
+from lms.djangoapps.courseware.exceptions import CourseAccessRedirect
 from lms.djangoapps.courseware.model_data import FieldDataCache
 from lms.djangoapps.courseware.module_render import get_module_for_descriptor
 from lms.djangoapps.courseware.courseware_access_exception import CoursewareAccessException
@@ -88,6 +89,18 @@ def test_get_course_func_with_access_error(self, course_access_func_name):
         assert error.value.access_response.error_code == 'not_visible_to_user'
         assert not error.value.access_response.has_access
 
+    @ddt.data(GET_COURSE_WITH_ACCESS, GET_COURSE_OVERVIEW_WITH_ACCESS)
+    def test_old_mongo_access_error(self, course_access_func_name):
+        course_access_func = self.COURSE_ACCESS_FUNCS[course_access_func_name]
+        user = UserFactory.create()
+        with self.store.default_store(ModuleStoreEnum.Type.mongo):
+            course = CourseFactory.create()
+
+        with pytest.raises(CourseAccessRedirect) as error:
+            course_access_func(user, 'load', course.id)
+        assert error.value.access_error.error_code == 'old_mongo'
+        assert not error.value.access_error.has_access
+
     @ddt.data(
         (GET_COURSE_WITH_ACCESS, 3),
         (GET_COURSE_OVERVIEW_WITH_ACCESS, 0),
diff --git a/openedx/features/course_experience/tests/views/test_course_home.py b/openedx/features/course_experience/tests/views/test_course_home.py
index 788d150360af..4859a46de041 100644
--- a/openedx/features/course_experience/tests/views/test_course_home.py
+++ b/openedx/features/course_experience/tests/views/test_course_home.py
@@ -527,6 +527,25 @@ def test_expired_course(self):
         )
         self.assertRedirects(response, expected_url)
 
+    def test_old_mongo_access_error(self):
+        """
+        Ensure that a user accessing an Old Mongo course sees a redirect to
+        the student dashboard, not a 404.
+        """
+        course = CourseFactory.create(default_store=ModuleStoreEnum.Type.mongo)
+        user = UserFactory(password=self.TEST_PASSWORD)
+        self.client.login(username=user.username, password=self.TEST_PASSWORD)
+
+        response = self.client.get(course_home_url(course))
+
+        expected_params = QueryDict(mutable=True)
+        expected_params['access_response_error'] = f'{course.display_name_with_default} is no longer available.'
+        expected_url = '{url}?{params}'.format(
+            url=reverse('dashboard'),
+            params=expected_params.urlencode(),
+        )
+        self.assertRedirects(response, expected_url)
+
     @mock.patch.dict(settings.FEATURES, {'DISABLE_START_DATES': False})
     def test_expiration_banner_with_expired_upgrade_deadline(self):
         """