diff --git a/docs/user/trusted-publishers/adding-a-publisher.md b/docs/user/trusted-publishers/adding-a-publisher.md
index 8f9643a0c24f..34498923df6c 100644
--- a/docs/user/trusted-publishers/adding-a-publisher.md
+++ b/docs/user/trusted-publishers/adding-a-publisher.md
@@ -117,6 +117,13 @@ each.
     **optionally** provide the name of a
     [GitLab CI/CD environment](https://docs.gitlab.com/ee/ci/environments/).
 
+    !!! note
+
+        The repository's namepsace and name must be provided as lowercase.
+        For example, if your project is at `https://gitlab.com/NameSpace/SampleProject`,
+        you must enter `namespace` as the namespace and `sampleproject` as the
+        project name.
+
     For example, if you have a project at `https://gitlab.com/namespace/sampleproject`
     with a top-level pipeline defined in `.gitlab-ci.yml` and a custom environment
     named `release`, then you'd do the following:
diff --git a/tests/unit/oidc/forms/test_gitlab.py b/tests/unit/oidc/forms/test_gitlab.py
index c6b494c06d11..f4c76179404a 100644
--- a/tests/unit/oidc/forms/test_gitlab.py
+++ b/tests/unit/oidc/forms/test_gitlab.py
@@ -152,6 +152,16 @@ def test_validate(self, data):
             },
             {"project": "some", "namespace": "some", "workflow_filepath": None},
             {"project": "some", "namespace": "some", "workflow_filepath": ""},
+            {
+                "namespace": "some-owner",
+                "project": "CaseSensitive",
+                "workflow_filepath": "some-workflow.yml",
+            },
+            {
+                "namespace": "CaseSensitive",
+                "project": "some-repo",
+                "workflow_filepath": "some-workflow.yml",
+            },
         ],
     )
     def test_validate_basic_invalid_fields(self, data):
diff --git a/warehouse/locale/messages.pot b/warehouse/locale/messages.pot
index e396b50bed4e..5530083fe7d1 100644
--- a/warehouse/locale/messages.pot
+++ b/warehouse/locale/messages.pot
@@ -394,7 +394,7 @@ msgid "Select project"
 msgstr ""
 
 #: warehouse/manage/forms.py:507 warehouse/oidc/forms/_core.py:36
-#: warehouse/oidc/forms/gitlab.py:47
+#: warehouse/oidc/forms/gitlab.py:60
 msgid "Specify project name"
 msgstr ""
 
@@ -677,7 +677,7 @@ msgid "Expired invitation for '${username}' deleted."
 msgstr ""
 
 #: warehouse/oidc/forms/_core.py:38 warehouse/oidc/forms/_core.py:49
-#: warehouse/oidc/forms/gitlab.py:50 warehouse/oidc/forms/gitlab.py:54
+#: warehouse/oidc/forms/gitlab.py:64 warehouse/oidc/forms/gitlab.py:68
 msgid "Invalid project name"
 msgstr ""
 
@@ -827,27 +827,31 @@ msgstr ""
 msgid "Name ends with .git or .atom"
 msgstr ""
 
-#: warehouse/oidc/forms/gitlab.py:31
+#: warehouse/oidc/forms/gitlab.py:34
+msgid "Value must be lowercase"
+msgstr ""
+
+#: warehouse/oidc/forms/gitlab.py:43
 msgid "Specify GitLab namespace (username or group/subgroup)"
 msgstr ""
 
-#: warehouse/oidc/forms/gitlab.py:36 warehouse/oidc/forms/gitlab.py:40
+#: warehouse/oidc/forms/gitlab.py:49 warehouse/oidc/forms/gitlab.py:53
 msgid "Invalid GitLab username or group/subgroup name."
 msgstr ""
 
-#: warehouse/oidc/forms/gitlab.py:62
+#: warehouse/oidc/forms/gitlab.py:76
 msgid "Specify top-level pipeline file path"
 msgstr ""
 
-#: warehouse/oidc/forms/gitlab.py:71
+#: warehouse/oidc/forms/gitlab.py:85
 msgid "Invalid environment name"
 msgstr ""
 
-#: warehouse/oidc/forms/gitlab.py:86
+#: warehouse/oidc/forms/gitlab.py:100
 msgid "Top-level pipeline file path must end with .yml or .yaml"
 msgstr ""
 
-#: warehouse/oidc/forms/gitlab.py:90
+#: warehouse/oidc/forms/gitlab.py:104
 msgid "Top-level pipeline file path cannot start or end with /"
 msgstr ""
 
diff --git a/warehouse/oidc/forms/gitlab.py b/warehouse/oidc/forms/gitlab.py
index 00b100cf5a51..2667d75092e6 100644
--- a/warehouse/oidc/forms/gitlab.py
+++ b/warehouse/oidc/forms/gitlab.py
@@ -22,6 +22,18 @@ def ends_with_atom_or_git(form: wtforms.Form, field: wtforms.Field) -> None:
         raise wtforms.validators.ValidationError(_("Name ends with .git or .atom"))
 
 
+def _lowercase_only(form: wtforms.Form, field: wtforms.Field) -> None:
+    """
+    GitLab API is case-insensitive for namespaces and projects.
+    Instead of changing the regular expressions to remove uppercase letters,
+    we add this additional validator to ensure that users only enter lowercase
+    characters, and provide a clear error message if they do not.
+    """
+    field_value = typing.cast(str, field.data)
+    if field_value != field_value.lower():
+        raise wtforms.validators.ValidationError(_("Value must be lowercase"))
+
+
 class GitLabPublisherBase(wtforms.Form):
     __params__ = ["namespace", "project", "workflow_filepath", "environment"]
 
@@ -31,6 +43,7 @@ class GitLabPublisherBase(wtforms.Form):
                 message=_("Specify GitLab namespace (username or group/subgroup)"),
             ),
             ends_with_atom_or_git,
+            _lowercase_only,
             wtforms.validators.Regexp(
                 _VALID_GITLAB_NAMESPACE,
                 message=_("Invalid GitLab username or group/subgroup name."),
@@ -46,6 +59,7 @@ class GitLabPublisherBase(wtforms.Form):
         validators=[
             wtforms.validators.InputRequired(message=_("Specify project name")),
             ends_with_atom_or_git,
+            _lowercase_only,
             wtforms.validators.Regexp(
                 _VALID_GITLAB_PROJECT, message=_("Invalid project name")
             ),