diff --git a/frontend/src/components/project/NewProjectPopup.tsx b/frontend/src/components/project/NewProjectPopup.tsx
index 512f1138db9..77290a26d5e 100644
--- a/frontend/src/components/project/NewProjectPopup.tsx
+++ b/frontend/src/components/project/NewProjectPopup.tsx
@@ -29,7 +29,7 @@ import {
   useTheme,
 } from '@mui/material';
 import { uniq } from 'lodash';
-import { ReactNode, useCallback, useMemo, useState } from 'react';
+import { ReactNode, useCallback, useEffect, useMemo, useState } from 'react';
 import { Trans, useTranslation } from 'react-i18next';
 import { useHistory } from 'react-router';
 import { useClustersConf } from '../../lib/k8s';
@@ -136,12 +136,35 @@ function ProjectFromExistingNamespace({ onBack }: { onBack: () => void }) {
   // Check if project name already exists (using normalized form to match existing entries)
   const projectNameExists =
     projectName.length > 0 && existingProjectNames.has(toKubernetesName(projectName));
+  const namespaceToProjectMap = useMemo(() => {
+    const map = new Map<string, string>();
+    if (!namespaces) return map;
+    namespaces.forEach(ns => {
+      const projectId = ns.metadata?.labels?.[PROJECT_ID_LABEL];
+      if (projectId) {
+        map.set(ns.metadata.name, projectId);
+      }
+    });
+    return map;
+  }, [namespaces]);
+
+  const effectiveNamespace = selectedNamespace || toKubernetesName(typedNamespace);
+  const isNamespaceAlreadyAssigned = effectiveNamespace
+    ? namespaceToProjectMap.has(effectiveNamespace)
+    : false;
 
   const isReadyToCreate =
     selectedClusters.length &&
     (selectedNamespace || typedNamespace) &&
     projectName &&
-    !projectNameExists;
+    !projectNameExists &&
+    !isNamespaceAlreadyAssigned;
+
+  useEffect(() => {
+    if (selectedNamespace && namespaceToProjectMap.has(selectedNamespace)) {
+      setSelectedNamespace(undefined);
+    }
+  }, [selectedNamespace, namespaceToProjectMap]);
 
   /**
    * Creates or updates namespaces for the project
@@ -285,6 +308,7 @@ function ProjectFromExistingNamespace({ onBack }: { onBack: () => void }) {
           onInputChange={(e, v) => {
             setTypedNamespace(v);
           }}
+          getOptionDisabled={option => namespaceToProjectMap.has(option)}
           renderInput={params => (
             <TextField
               {...params}