diff --git a/core/middlewares/middlewares.py b/core/middlewares/middlewares.py
index d5b21beb..43bdb605 100644
--- a/core/middlewares/middlewares.py
+++ b/core/middlewares/middlewares.py
@@ -2,7 +2,9 @@
 import time
 
 import requests
+from django.conf import settings
 from django.http import HttpResponseNotFound, HttpResponse
+from django.http.response import JsonResponse
 from django.utils.deprecation import MiddlewareMixin
 from request_logging.middleware import LoggingMiddleware
 from rest_framework.views import APIView
@@ -52,7 +54,6 @@ class ResponseHeadersMiddleware(BaseMiddleware):
     def __call__(self, request):
         start_time = time.time()
         response = self.get_response(request)
-        from django.conf import settings
         response[VERSION_HEADER] = settings.VERSION
         try:
             response[REQUEST_USER_HEADER] = str(getattr(request, 'user', None))
@@ -89,6 +90,109 @@ def __call__(self, request):
         return response
 
 
+class RequireAuthenticationMiddleware(BaseMiddleware):
+    """Block anonymous API access unless the request matches an approved bypass."""
+
+    exempt_path_prefixes = (
+        '/healthcheck/',
+        '/users/api-token/',
+        '/users/login/',
+        '/users/logout/',
+        '/users/signup/',
+        '/users/password/reset/',
+        '/users/oidc/',
+        '/oidc/',
+        '/fhir/',
+        '/swagger',
+        '/redoc/',
+        '/admin/',
+    )
+    exempt_exact_paths = {
+        '/',
+        '/version',
+        '/changelog',
+        '/feedback',
+        '/toggles',
+        '/locales',
+        '/events',
+    }
+    forbidden_response = {
+        'detail': 'Authentication required. Anonymous API access is disabled.',
+        'upgrade_url': 'https://app.openconceptlab.org/pricing',
+    }
+
+    def __call__(self, request):
+        """Allow exempt and approved anonymous traffic, otherwise return 403."""
+        if self.is_request_allowed(request):
+            return self.get_response(request)
+
+        return JsonResponse(self.forbidden_response, status=403)
+
+    def is_request_allowed(self, request):
+        """Return whether the current request can bypass authentication enforcement."""
+        if request.method == 'OPTIONS' or request.META.get('HTTP_USER_AGENT', '').startswith('ELB-HealthChecker'):
+            return True
+
+        user = getattr(request, 'user', None)
+        return getattr(user, 'is_authenticated', False) or self.is_exempt_path(request.path) or \
+            self.has_approved_client_header(request) or \
+            self.has_approved_api_key(request) or self.has_approved_ip(request)
+
+    @classmethod
+    def is_exempt_path(cls, path):
+        """Return whether a request path must remain accessible to anonymous users."""
+        normalized_path = path.rstrip('/') or '/'
+        if normalized_path in cls.exempt_exact_paths:
+            return True
+
+        if any(path.startswith(prefix) for prefix in cls.exempt_path_prefixes):
+            return True
+
+        return (
+            path.startswith('/users/')
+            and (
+                '/verify/' in path
+                or path.endswith('/sso-migrate/')
+                or path.endswith('/following/')
+            )
+        )
+
+    @staticmethod
+    def has_approved_client_header(request):
+        """Match the configured anonymous allowlist against the X-OCL-CLIENT header."""
+        client_name = request.META.get('HTTP_X_OCL_CLIENT', '').strip()
+        return bool(client_name and client_name in settings.APPROVED_ANONYMOUS_CLIENTS)
+
+    @staticmethod
+    def has_approved_api_key(request):
+        """Match allowlisted anonymous API keys from common request locations."""
+        approved_keys = settings.APPROVED_ANONYMOUS_API_KEYS
+        if not approved_keys:
+            return False
+
+        authorization = request.META.get('HTTP_AUTHORIZATION', '').strip()
+        x_api_key = request.META.get('HTTP_X_API_KEY', '').strip()
+        tokens = [authorization, x_api_key]
+        bearer_token = authorization.split(None, 1)[1].strip() if ' ' in authorization else ''
+        if bearer_token:
+            tokens.append(bearer_token)
+
+        return any(token and token in approved_keys for token in tokens)
+
+    @staticmethod
+    def has_approved_ip(request):
+        """Match source IPs using the socket address only."""
+        approved_ips = settings.APPROVED_ANONYMOUS_IPS
+        if not approved_ips:
+            return False
+
+        remote_addr = request.META.get('REMOTE_ADDR', '').strip()
+        if not remote_addr:
+            return False
+
+        return remote_addr in approved_ips
+
+
 class FhirMiddleware(BaseMiddleware):
     """
     It is used to expose FHIR endpoints under FHIR subdomain only and convert content from xml to json.
@@ -98,7 +202,6 @@ class FhirMiddleware(BaseMiddleware):
     def __call__(self, request):
         absolute_uri = request.build_absolute_uri()
 
-        from django.conf import settings
         if settings.FHIR_SUBDOMAIN:
             uri = absolute_uri.split('/')
             domain = uri[2] if len(uri) > 2 else ''
diff --git a/core/middlewares/tests.py b/core/middlewares/tests.py
new file mode 100644
index 00000000..3f3ef360
--- /dev/null
+++ b/core/middlewares/tests.py
@@ -0,0 +1,167 @@
+import json
+
+from django.conf import settings
+from django.contrib.auth.models import AnonymousUser
+from django.http import HttpResponse
+from django.test import RequestFactory, SimpleTestCase, override_settings
+
+from core.middlewares.middlewares import RequireAuthenticationMiddleware
+
+
+@override_settings(
+    REQUIRE_AUTHENTICATION=True,
+    APPROVED_ANONYMOUS_CLIENTS=['test-client'],
+    APPROVED_ANONYMOUS_API_KEYS=['test-api-key'],
+    APPROVED_ANONYMOUS_IPS=['10.0.0.1'],
+)
+class RequireAuthenticationMiddlewareTest(SimpleTestCase):
+    """Verify anonymous authentication enforcement and approved bypasses."""
+
+    def setUp(self):
+        """Create a request factory and middleware instance for each test."""
+        self.factory = RequestFactory()
+        self.middleware = RequireAuthenticationMiddleware(lambda request: HttpResponse('ok'))
+
+    def make_request(self, path='/orgs/', method='get', user=None, **meta):
+        """Build a request object with a controllable authenticated user state."""
+        request_method = getattr(self.factory, method.lower())
+        request = request_method(path, **meta)
+        request.user = user or AnonymousUser()
+        return request
+
+    def test_allows_authenticated_request(self):
+        """Authenticated requests should bypass the anonymous access gate."""
+        user = type('AuthenticatedUser', (), {'is_authenticated': True})()
+
+        response = self.middleware(self.make_request(user=user))
+
+        self.assertEqual(response.status_code, 200)
+
+    def test_blocks_anonymous_request_for_protected_path(self):
+        """Anonymous traffic to protected API paths should receive a 403 response."""
+        response = self.middleware(self.make_request('/orgs/OCL/'))
+
+        self.assertEqual(response.status_code, 403)
+        self.assertEqual(
+            json.loads(response.content),
+            {
+                'detail': 'Authentication required. Anonymous API access is disabled.',
+                'upgrade_url': 'https://app.openconceptlab.org/pricing',
+            }
+        )
+
+    def test_allows_anonymous_request_for_approved_client_header(self):
+        """Approved X-OCL-CLIENT values should retain anonymous access."""
+        response = self.middleware(self.make_request('/orgs/OCL/', HTTP_X_OCL_CLIENT='test-client'))
+
+        self.assertEqual(response.status_code, 200)
+
+    def test_blocks_anonymous_request_for_unapproved_client_header(self):
+        """Unknown X-OCL-CLIENT values should still be rejected."""
+        response = self.middleware(self.make_request('/orgs/OCL/', HTTP_X_OCL_CLIENT='unknown-client'))
+
+        self.assertEqual(response.status_code, 403)
+
+    def test_blocks_anonymous_request_for_whitespace_client_header(self):
+        """Whitespace-only client header values should be rejected after normalization."""
+        response = self.middleware(self.make_request('/orgs/OCL/', HTTP_X_OCL_CLIENT='   '))
+
+        self.assertEqual(response.status_code, 403)
+
+    def test_allows_anonymous_request_for_approved_api_key_header(self):
+        """Allowlisted anonymous API keys should bypass the gate."""
+        response = self.middleware(self.make_request('/orgs/OCL/', HTTP_X_API_KEY='test-api-key'))
+
+        self.assertEqual(response.status_code, 200)
+
+    def test_allows_anonymous_request_for_approved_authorization_token(self):
+        """Allowlisted bearer or token credentials should bypass the gate."""
+        response = self.middleware(self.make_request('/orgs/OCL/', HTTP_AUTHORIZATION='Token test-api-key'))
+
+        self.assertEqual(response.status_code, 200)
+
+    def test_blocks_anonymous_request_for_query_string_api_key(self):
+        """Query string API keys should not bypass the authentication gate."""
+        response = self.middleware(self.make_request('/orgs/OCL/?api_key=test-api-key'))
+
+        self.assertEqual(response.status_code, 403)
+
+    def test_allows_anonymous_request_for_approved_ip(self):
+        """Allowlisted source IPs should keep anonymous access."""
+        response = self.middleware(self.make_request('/orgs/OCL/', REMOTE_ADDR='10.0.0.1'))
+
+        self.assertEqual(response.status_code, 200)
+
+    def test_blocks_anonymous_request_for_forwarded_ip_only(self):
+        """Forwarded IP headers alone should not bypass the authentication gate."""
+        response = self.middleware(
+            self.make_request('/orgs/OCL/', HTTP_X_FORWARDED_FOR='10.0.0.1', REMOTE_ADDR='203.0.113.5')
+        )
+
+        self.assertEqual(response.status_code, 403)
+
+    def test_allows_options_request(self):
+        """CORS preflight requests should not be blocked."""
+        response = self.middleware(self.make_request('/orgs/OCL/', method='options'))
+
+        self.assertEqual(response.status_code, 200)
+
+    def test_allows_elb_health_checker_request(self):
+        """Infrastructure health checks should bypass the gate."""
+        response = self.middleware(
+            self.make_request('/orgs/OCL/', HTTP_USER_AGENT='ELB-HealthChecker/2.0')
+        )
+
+        self.assertEqual(response.status_code, 200)
+
+    def test_allows_exempt_exact_paths(self):
+        """Public root-level utility endpoints should remain anonymous."""
+        for path in ['/', '/version/', '/changelog/', '/feedback/', '/toggles/', '/locales/', '/events/']:
+            with self.subTest(path=path):
+                response = self.middleware(self.make_request(path))
+                self.assertEqual(response.status_code, 200)
+
+    def test_allows_exempt_path_prefixes(self):
+        """Auth, docs, admin, and FHIR prefixes should remain anonymous."""
+        paths = [
+            '/healthcheck/',
+            '/users/api-token/',
+            '/users/login/',
+            '/users/logout/',
+            '/users/signup/',
+            '/users/password/reset/',
+            '/users/oidc/code-exchange/',
+            '/oidc/authenticate/',
+            '/fhir/',
+            '/swagger/',
+            '/swagger.json',
+            '/redoc/',
+            '/admin/login/',
+        ]
+
+        for path in paths:
+            with self.subTest(path=path):
+                response = self.middleware(self.make_request(path))
+                self.assertEqual(response.status_code, 200)
+
+    def test_allows_exempt_dynamic_user_paths(self):
+        """Public user verification and following endpoints should remain anonymous."""
+        paths = [
+            '/users/alice/verify/token-123/',
+            '/users/alice/sso-migrate/',
+            '/users/alice/following/',
+        ]
+
+        for path in paths:
+            with self.subTest(path=path):
+                response = self.middleware(self.make_request(path))
+                self.assertEqual(response.status_code, 200)
+
+
+@override_settings(REQUIRE_AUTHENTICATION=False)
+class RequireAuthenticationSettingsTest(SimpleTestCase):
+    """Verify auth-enforcement middleware configuration toggles cleanly."""
+
+    def test_authentication_middleware_not_inserted_when_disabled(self):
+        """RequireAuthenticationMiddleware should be absent when the feature is disabled."""
+        self.assertNotIn('core.middlewares.middlewares.RequireAuthenticationMiddleware', settings.MIDDLEWARE)
diff --git a/core/settings.py b/core/settings.py
index 254d1783..7424412d 100644
--- a/core/settings.py
+++ b/core/settings.py
@@ -49,6 +49,17 @@
 else:
     ENABLE_THROTTLING = os.environ.get('ENABLE_THROTTLING', False) in ['true', 'True', 'TRUE', True]
 
+
+def get_set_from_env(name):
+    """Return a trimmed set for comma-separated environment variables."""
+    return {value.strip() for value in os.environ.get(name, '').split(',') if value.strip()}
+
+
+REQUIRE_AUTHENTICATION = os.environ.get('REQUIRE_AUTHENTICATION', 'false').lower() in ['true', '1']
+APPROVED_ANONYMOUS_CLIENTS = get_set_from_env('APPROVED_ANONYMOUS_CLIENTS')
+APPROVED_ANONYMOUS_API_KEYS = get_set_from_env('APPROVED_ANONYMOUS_API_KEYS')
+APPROVED_ANONYMOUS_IPS = get_set_from_env('APPROVED_ANONYMOUS_IPS')
+
 ALLOWED_HOSTS = ['*']
 
 CORS_ALLOW_HEADERS = default_headers + (
@@ -199,6 +210,11 @@
     }
     MIDDLEWARE = [*MIDDLEWARE, 'core.middlewares.middlewares.ThrottleHeadersMiddleware']
 
+if REQUIRE_AUTHENTICATION:
+    auth_middleware = 'django.contrib.auth.middleware.AuthenticationMiddleware'
+    auth_index = MIDDLEWARE.index(auth_middleware)
+    MIDDLEWARE.insert(auth_index + 1, 'core.middlewares.middlewares.RequireAuthenticationMiddleware')
+
 
 ROOT_URLCONF = 'core.urls'