diff --git a/app/main/views/organisations.py b/app/main/views/organisations.py
index 7b656bbfa..01c0224d4 100644
--- a/app/main/views/organisations.py
+++ b/app/main/views/organisations.py
@@ -448,13 +448,24 @@ def edit_organisation_domains(org_id):
     form = OrganisationDomainsForm()
 
     if form.validate_on_submit():
-        organisations_client.update_organisation(
-            org_id,
-            domains=list(OrderedDict.fromkeys(
-                domain.lower()
-                for domain in filter(None, form.domains.data)
-            )),
-        )
+        try:
+            organisations_client.update_organisation(
+                org_id,
+                domains=list(OrderedDict.fromkeys(
+                    domain.lower()
+                    for domain in filter(None, form.domains.data)
+                )),
+            )
+        except HTTPError as e:
+            error_message = "Domain already exists"
+            if e.status_code == 400 and error_message in e.message:
+                flash("This domain is already in use", "error")
+                return render_template(
+                    'views/organisations/organisation/settings/edit-domains.html',
+                    form=form,
+                )
+            else:
+                raise e
         return redirect(url_for('.organisation_settings', org_id=org_id))
 
     form.populate(current_organisation.domains)
@@ -483,7 +494,7 @@ def confirm_edit_organisation_name(org_id):
         except HTTPError as e:
             error_msg = "Organisation name already exists"
             if e.status_code == 400 and error_msg in e.message:
-                # Redirect the user back to the change service name screen
+                # Redirect the user back to the change organisation name screen
                 flash('This organisation name is already in use', 'error')
                 return redirect(url_for('main.edit_organisation_name', org_id=org_id))
             else:
diff --git a/tests/app/main/views/organisations/test_organisation.py b/tests/app/main/views/organisations/test_organisation.py
index 6569e4acf..dbc8409e6 100644
--- a/tests/app/main/views/organisations/test_organisation.py
+++ b/tests/app/main/views/organisations/test_organisation.py
@@ -991,6 +991,38 @@ def test_update_organisation_domains(
     )
 
 
+def test_update_organisation_domains_when_domain_already_exists(
+    mocker,
+    client_request,
+    fake_uuid,
+    organisation_one,
+    mock_get_organisation,
+):
+    user = create_platform_admin_user()
+    client_request.login(user)
+
+    mocker.patch('app.organisations_client.update_organisation', side_effect=HTTPError(
+        response=Mock(
+            status_code=400,
+            json={'result': 'error', 'message': 'Domain already exists'}
+        ),
+        message="Domain already exists")
+    )
+
+    response = client_request.post(
+        'main.edit_organisation_domains',
+        org_id=ORGANISATION_ID,
+        _data={
+            'domains': [
+                'example.gov.uk',
+            ]
+        },
+        _expected_status=200,
+    )
+
+    assert response.find("div", class_="banner-dangerous").text.strip() == "This domain is already in use"
+
+
 def test_update_organisation_name(
     platform_admin_client,
     organisation_one,