diff --git a/moto/cognitoidp/models.py b/moto/cognitoidp/models.py
index a8af41c40..fa34b20c5 100644
--- a/moto/cognitoidp/models.py
+++ b/moto/cognitoidp/models.py
@@ -1403,10 +1403,13 @@ class CognitoIdpBackend(BaseBackend):
         return resource_server
 
     def sign_up(self, client_id, username, password, attributes):
+        # This method may not be authenticated - which means we don't know which region the request was send to
+        # Let's cycle through all regions to find out which one contains our client_id
         user_pool = None
-        for p in self.user_pools.values():
-            if client_id in p.clients:
-                user_pool = p
+        for backend in cognitoidp_backends.values():
+            for p in backend.user_pools.values():
+                if client_id in p.clients:
+                    user_pool = p
         if user_pool is None:
             raise ResourceNotFoundError(client_id)
         elif user_pool._get_user(username):
diff --git a/tests/test_cognitoidp/test_server.py b/tests/test_cognitoidp/test_server.py
new file mode 100644
index 000000000..c904058cd
--- /dev/null
+++ b/tests/test_cognitoidp/test_server.py
@@ -0,0 +1,57 @@
+import json
+import moto.server as server
+
+
+def test_sign_up_method_without_authentication():
+    backend = server.create_backend_app("cognito-idp")
+    test_client = backend.test_client()
+
+    # Create User Pool
+    res = test_client.post(
+        "/",
+        data='{"PoolName": "test-pool"}',
+        headers={
+            "X-Amz-Target": "AWSCognitoIdentityProviderService.CreateUserPool",
+            "Authorization": "AWS4-HMAC-SHA256 Credential=abcd/20010101/us-east-2/s3/aws4_request, SignedHeaders=host;x-amz-content-sha256;x-amz-date, Signature=...",
+        },
+    )
+    user_pool_id = json.loads(res.data)["UserPool"]["Id"]
+
+    # Create User Pool Client
+    data = {
+        "UserPoolId": user_pool_id,
+        "ClientName": "some-client",
+        "GenerateSecret": False,
+        "ExplicitAuthFlows": ["ALLOW_USER_PASSWORD_AUTH"],
+    }
+    res = test_client.post(
+        "/",
+        data=json.dumps(data),
+        headers={
+            "X-Amz-Target": "AWSCognitoIdentityProviderService.CreateUserPoolClient",
+            "Authorization": "AWS4-HMAC-SHA256 Credential=abcd/20010101/us-east-2/s3/aws4_request, SignedHeaders=host;x-amz-content-sha256;x-amz-date, Signature=...",
+        },
+    )
+    client_id = json.loads(res.data)["UserPoolClient"]["ClientId"]
+
+    # List User Pool Clients, to verify it exists
+    data = {"UserPoolId": user_pool_id}
+    res = test_client.post(
+        "/",
+        data=json.dumps(data),
+        headers={
+            "X-Amz-Target": "AWSCognitoIdentityProviderService.ListUserPoolClients",
+            "Authorization": "AWS4-HMAC-SHA256 Credential=abcd/20010101/us-east-2/s3/aws4_request, SignedHeaders=host;x-amz-content-sha256;x-amz-date, Signature=...",
+        },
+    )
+    json.loads(res.data)["UserPoolClients"].should.have.length_of(1)
+
+    # Sign Up User
+    data = {"ClientId": client_id, "Username": "test@gmail.com", "Password": "12345678"}
+    res = test_client.post(
+        "/",
+        data=json.dumps(data),
+        headers={"X-Amz-Target": "AWSCognitoIdentityProviderService.SignUp"},
+    )
+    res.status_code.should.equal(200)
+    json.loads(res.data).should.have.key("UserConfirmed").equals(False)