diff --git a/CHANGELOG.md b/CHANGELOG.md
index 79bc5a87..ec2f664a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,8 @@ and this project adheres to
 
 - ♿️(frontend) Focus main container after navigation #1854
 - 🚸(backend) sort user search results by proximity with the active user #1802 
+- 🚸(oidc) ignore case when fallback on email #1880
+
 
 ### Fixed
 
diff --git a/src/backend/core/models.py b/src/backend/core/models.py
index 69fc24f2..7b3a2dc2 100644
--- a/src/backend/core/models.py
+++ b/src/backend/core/models.py
@@ -118,11 +118,11 @@ class UserManager(auth_models.UserManager):
 
             if settings.OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION:
                 try:
-                    return self.get(email=email)
+                    return self.get(email__iexact=email)
                 except self.model.DoesNotExist:
                     pass
             elif (
-                self.filter(email=email).exists()
+                self.filter(email__iexact=email).exists()
                 and not settings.OIDC_ALLOW_DUPLICATE_EMAILS
             ):
                 raise DuplicateEmailError(
@@ -1905,7 +1905,7 @@ class Invitation(BaseModel):
 
         # Check if an identity already exists for the provided email
         if (
-            User.objects.filter(email=self.email).exists()
+            User.objects.filter(email__iexact=self.email).exists()
             and not settings.OIDC_ALLOW_DUPLICATE_EMAILS
         ):
             raise ValidationError(
diff --git a/src/backend/core/tests/authentication/test_backends.py b/src/backend/core/tests/authentication/test_backends.py
index 6d2d1ad1..0ff0a78a 100644
--- a/src/backend/core/tests/authentication/test_backends.py
+++ b/src/backend/core/tests/authentication/test_backends.py
@@ -68,6 +68,30 @@ def test_authentication_getter_existing_user_via_email(
     assert user == db_user
 
 
+def test_authentication_getter_existing_user_via_email_case_insensitive(
+    django_assert_num_queries, monkeypatch
+):
+    """
+    If an existing user doesn't match the sub but matches the email with different case,
+    the user should be returned (case-insensitive email matching).
+    """
+
+    klass = OIDCAuthenticationBackend()
+    db_user = UserFactory(email="john.doe@example.com")
+
+    def get_userinfo_mocked(*args):
+        return {"sub": "123", "email": "JOHN.DOE@EXAMPLE.COM"}
+
+    monkeypatch.setattr(OIDCAuthenticationBackend, "get_userinfo", get_userinfo_mocked)
+
+    with django_assert_num_queries(4):  # user by sub, user by mail, update sub
+        user = klass.get_or_create_user(
+            access_token="test-token", id_token=None, payload=None
+        )
+
+    assert user == db_user
+
+
 def test_authentication_getter_email_none(monkeypatch):
     """
     If no user is found with the sub and no email is provided, a new user should be created.
@@ -157,6 +181,39 @@ def test_authentication_getter_existing_user_no_fallback_to_email_no_duplicate(
     assert models.User.objects.count() == 1
 
 
+def test_authentication_getter_existing_user_no_fallback_to_email_no_duplicate_case_insensitive(
+    settings, monkeypatch
+):
+    """
+    When the "OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION" setting is set to False,
+    the system should detect duplicate emails even with different case.
+    """
+
+    klass = OIDCAuthenticationBackend()
+    _db_user = UserFactory(email="john.doe@example.com")
+
+    # Set the setting to False
+    settings.OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION = False
+    settings.OIDC_ALLOW_DUPLICATE_EMAILS = False
+
+    def get_userinfo_mocked(*args):
+        return {"sub": "123", "email": "JOHN.DOE@EXAMPLE.COM"}
+
+    monkeypatch.setattr(OIDCAuthenticationBackend, "get_userinfo", get_userinfo_mocked)
+
+    with pytest.raises(
+        SuspiciousOperation,
+        match=(
+            "We couldn't find a user with this sub but the email is already associated "
+            "with a registered user."
+        ),
+    ):
+        klass.get_or_create_user(access_token="test-token", id_token=None, payload=None)
+
+    # Since the sub doesn't match, it should not create a new user
+    assert models.User.objects.count() == 1
+
+
 def test_authentication_getter_existing_user_with_email(
     django_assert_num_queries, monkeypatch
 ):
diff --git a/src/backend/core/tests/documents/test_api_document_invitations.py b/src/backend/core/tests/documents/test_api_document_invitations.py
index 46428c6f..94eda072 100644
--- a/src/backend/core/tests/documents/test_api_document_invitations.py
+++ b/src/backend/core/tests/documents/test_api_document_invitations.py
@@ -596,6 +596,38 @@ def test_api_document_invitations_create_cannot_invite_existing_users():
     }
 
 
+def test_api_item_invitations_create_cannot_invite_existing_users_case_insensitive():
+    """
+    It should not be possible to invite already existing users, even with different email case.
+    """
+    user = factories.UserFactory()
+    document = factories.DocumentFactory(users=[(user, "owner")])
+    existing_user = factories.UserFactory()
+
+    # Build an invitation to the email of an existing identity with different case
+    invitation_values = {
+        "email": existing_user.email.upper(),
+        "role": random.choice(models.RoleChoices.values),
+    }
+
+    client = APIClient()
+    client.force_login(user)
+
+    client = APIClient()
+    client.force_login(user)
+
+    response = client.post(
+        f"/api/v1.0/documents/{document.id!s}/invitations/",
+        invitation_values,
+        format="json",
+    )
+
+    assert response.status_code == 400
+    assert response.json() == {
+        "email": ["This email is already associated to a registered user."]
+    }
+
+
 def test_api_document_invitations_create_lower_email():
     """
     No matter the case, the email should be converted to lowercase.