diff --git a/admin_site/tests.py b/admin_site/tests.py
index 3a699893..eb844a55 100644
--- a/admin_site/tests.py
+++ b/admin_site/tests.py
@@ -8,10 +8,12 @@
 from social_core.backends.base import BaseAuth
 
 from paths.const import INSTANCE_SUPER_ADMIN_ROLE
+from paths.context import RealmContext, realm_context
 
 from admin_site.api import check_user_in_other_clusters
 from admin_site.auth_backends import NZCPortalOAuth2
 from admin_site.auth_pipeline import assign_roles
+from admin_site.wagtail_hooks import instance_chooser
 from frameworks.models import FrameworkConfig
 from frameworks.tests.factories import FrameworkFactory
 from nodes.tests.factories import InstanceConfigFactory
@@ -100,6 +102,51 @@ def post(*_args: object, **_kwargs: object) -> Never:
     assert check_user_in_other_clusters('user@example.com', request) is None
 
 
+def _chooser_labels(user, realm, rf) -> set[str]:
+    request = rf.get('/admin/')
+    request.user = user
+    ctx = RealmContext(realm=realm, user=user)
+    with realm_context.activate(ctx):
+        items = instance_chooser.menu_items_for_request(request)
+    return {item.label for item in items}
+
+
+def test_instance_chooser_omits_hidden_instances(rf) -> None:
+    admin = UserFactory.create(is_staff=True, is_superuser=True)
+    visible_a = InstanceConfigFactory.create(identifier='visible-a', name='Visible A')
+    InstanceConfigFactory.create(identifier='visible-b', name='Visible B')
+    InstanceConfigFactory.create(identifier='hidden-one', name='Hidden One', is_hidden=True)
+
+    labels = _chooser_labels(admin, visible_a, rf)
+
+    assert 'Visible A' in labels
+    assert 'Visible B' in labels
+    assert 'Hidden One' not in labels
+
+
+def test_instance_chooser_keeps_active_hidden_instance(rf) -> None:
+    # A user currently on a hidden instance must still see it (and be able to
+    # switch away), so the active realm is exempt from the filter.
+    admin = UserFactory.create(is_staff=True, is_superuser=True)
+    InstanceConfigFactory.create(identifier='visible', name='Visible')
+    hidden = InstanceConfigFactory.create(identifier='hidden', name='Hidden', is_hidden=True)
+
+    labels = _chooser_labels(admin, hidden, rf)
+
+    assert 'Hidden' in labels
+    assert 'Visible' in labels
+
+
+def test_hidden_instance_still_reachable() -> None:
+    # The hiding is listing-only: it does not touch get_adminable_instances(),
+    # which is the authorization gate for directly switching to an instance.
+    admin = UserFactory.create(is_staff=True, is_superuser=True)
+    hidden = InstanceConfigFactory.create(identifier='hidden', name='Hidden', is_hidden=True)
+
+    assert admin.user_is_admin_for_instance(hidden)
+    assert hidden in admin.get_adminable_instances()
+
+
 def test_nzcportal_city_admin_maps_to_instance_super_admin() -> None:
     details = NZCPortalOAuth2(strategy=None)._get_user_details(
         {
diff --git a/admin_site/wagtail_hooks.py b/admin_site/wagtail_hooks.py
index a006ff0a..4b2d7247 100644
--- a/admin_site/wagtail_hooks.py
+++ b/admin_site/wagtail_hooks.py
@@ -2,6 +2,7 @@
 
 from typing import TYPE_CHECKING
 
+from django.db.models import Q
 from django.templatetags.static import static
 from django.urls import reverse
 from django.utils.html import format_html
@@ -60,7 +61,12 @@ class InstanceItem(MenuItem):
 class InstanceChooserMenu(Menu):
     def menu_items_for_request(self, request: HttpRequest):
         user = user_or_bust(request.user)
-        instances = user.get_adminable_instances()
+        # Hide instances flagged as ``is_hidden`` from the chooser, but keep the
+        # currently-active one visible so a user on a hidden instance can switch
+        # away. This filters the listing only; ``get_adminable_instances()`` (the
+        # authorization gate for direct switching) is deliberately left untouched.
+        current = realm_context.get().realm
+        instances = user.get_adminable_instances().filter(Q(is_hidden=False) | Q(pk=current.pk if current is not None else None))
         if len(instances) < 2:
             return []
         items = []
diff --git a/nodes/migrations/0050_instanceconfig_is_hidden.py b/nodes/migrations/0050_instanceconfig_is_hidden.py
new file mode 100644
index 00000000..3ebae493
--- /dev/null
+++ b/nodes/migrations/0050_instanceconfig_is_hidden.py
@@ -0,0 +1,18 @@
+# Generated by Django 6.0.3 on 2026-06-18 11:10
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('nodes', '0049_instanceconfig_copy_of_nodeconfig_copy_of'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='instanceconfig',
+            name='is_hidden',
+            field=models.BooleanField(default=False, help_text='Hide this instance from the admin instance chooser. It remains reachable directly and via permissions.'),
+        ),
+    ]
diff --git a/nodes/models.py b/nodes/models.py
index 1b4ace2b..3676a19e 100644
--- a/nodes/models.py
+++ b/nodes/models.py
@@ -472,6 +472,10 @@ class InstanceConfig(
         default=False,
         help_text=_('Whether end-user mutation surfaces should treat this instance as read-only.'),
     )
+    is_hidden = models.BooleanField(
+        default=False,
+        help_text=_('Hide this instance from the admin instance chooser. It remains reachable directly and via permissions.'),
+    )
 
     created_at = models.DateTimeField(default=timezone.now)
     modified_at = models.DateTimeField(auto_now=True)