diff --git a/src/main/java/net/siegeln/cameleer/saas/onboarding/OnboardingController.java b/src/main/java/net/siegeln/cameleer/saas/onboarding/OnboardingController.java
index df394b0..bff31fc 100644
--- a/src/main/java/net/siegeln/cameleer/saas/onboarding/OnboardingController.java
+++ b/src/main/java/net/siegeln/cameleer/saas/onboarding/OnboardingController.java
@@ -1,15 +1,20 @@
 package net.siegeln.cameleer.saas.onboarding;
 
 import jakarta.validation.Valid;
+import java.util.Map;
 import net.siegeln.cameleer.saas.tenant.TenantEntity;
 import net.siegeln.cameleer.saas.tenant.dto.TenantResponse;
+import net.siegeln.cameleer.saas.tenant.TenantRepository;
+import net.siegeln.cameleer.saas.tenant.TenantStatus;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
 import org.springframework.security.core.annotation.AuthenticationPrincipal;
 import org.springframework.security.oauth2.jwt.Jwt;
+import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.PostMapping;
 import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RequestParam;
 import org.springframework.web.bind.annotation.RestController;
 
 @RestController
@@ -17,9 +22,11 @@ import org.springframework.web.bind.annotation.RestController;
 public class OnboardingController {
 
     private final OnboardingService onboardingService;
+    private final TenantRepository tenantRepository;
 
-    public OnboardingController(OnboardingService onboardingService) {
+    public OnboardingController(OnboardingService onboardingService, TenantRepository tenantRepository) {
         this.onboardingService = onboardingService;
+        this.tenantRepository = tenantRepository;
     }
 
     public record CreateTrialTenantRequest(
@@ -35,13 +42,23 @@ public class OnboardingController {
             String slug
     ) {}
 
+    @GetMapping("/slug-available")
+    public ResponseEntity<Map<String, Boolean>> slugAvailable(@RequestParam String slug) {
+        boolean taken = tenantRepository.existsBySlugAndStatusNot(slug, TenantStatus.DELETED);
+        return ResponseEntity.ok(Map.of("available", !taken));
+    }
+
     @PostMapping("/tenant")
     public ResponseEntity<TenantResponse> createTrialTenant(
             @Valid @RequestBody CreateTrialTenantRequest request,
             @AuthenticationPrincipal Jwt jwt) {
 
         String userId = jwt.getSubject();
-        TenantEntity tenant = onboardingService.createTrialTenant(request.name(), request.slug(), userId);
-        return ResponseEntity.status(HttpStatus.CREATED).body(TenantResponse.from(tenant));
+        try {
+            TenantEntity tenant = onboardingService.createTrialTenant(request.name(), request.slug(), userId);
+            return ResponseEntity.status(HttpStatus.CREATED).body(TenantResponse.from(tenant));
+        } catch (IllegalArgumentException e) {
+            return ResponseEntity.status(HttpStatus.CONFLICT).build();
+        }
     }
 }
diff --git a/ui/src/pages/OnboardingPage.tsx b/ui/src/pages/OnboardingPage.tsx
index cbd8dd6..930165c 100644
--- a/ui/src/pages/OnboardingPage.tsx
+++ b/ui/src/pages/OnboardingPage.tsx
@@ -1,4 +1,4 @@
-import { useState } from 'react';
+import { useState, useEffect, useRef } from 'react';
 import { Card, Input, Button, FormField, Alert } from '@cameleer/design-system';
 import cameleerLogo from '@cameleer/design-system/assets/cameleer-logo.svg';
 import { api } from '../api/client';
@@ -16,9 +16,32 @@ export function OnboardingPage() {
   const [name, setName] = useState('');
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState<string | null>(null);
+  const [slugAvailable, setSlugAvailable] = useState<boolean | null>(null);
+  const [checkingSlug, setCheckingSlug] = useState(false);
+  const debounceRef = useRef<ReturnType<typeof setTimeout>>(undefined);
 
   const slug = toSlug(name);
 
+  useEffect(() => {
+    setSlugAvailable(null);
+    if (!slug) return;
+
+    setCheckingSlug(true);
+    clearTimeout(debounceRef.current);
+    debounceRef.current = setTimeout(async () => {
+      try {
+        const res = await api.get<{ available: boolean }>(`/onboarding/slug-available?slug=${encodeURIComponent(slug)}`);
+        setSlugAvailable(res.available);
+      } catch {
+        setSlugAvailable(null);
+      } finally {
+        setCheckingSlug(false);
+      }
+    }, 400);
+
+    return () => clearTimeout(debounceRef.current);
+  }, [slug]);
+
   async function handleSubmit(e: React.FormEvent) {
     e.preventDefault();
     setError(null);
@@ -29,7 +52,12 @@ export function OnboardingPage() {
       // picks up the new org membership and scopes on the next token refresh.
       window.location.href = '/platform/';
     } catch (err) {
-      setError(err instanceof Error ? err.message : 'Failed to create tenant');
+      const msg = err instanceof Error ? err.message : String(err);
+      if (msg.includes('409')) {
+        setError('This organization name is already taken. Try a different organization name.');
+      } else {
+        setError(msg || 'Failed to create tenant');
+      }
       setLoading(false);
     }
   }
@@ -67,7 +95,13 @@ export function OnboardingPage() {
               </FormField>
 
               <FormField label="URL slug" htmlFor="onboard-slug" required
-                hint="Auto-generated from name. Appears in your dashboard URL."
+                hint={
+                  !slug ? 'Auto-generated from name. Appears in your dashboard URL.'
+                    : checkingSlug ? 'Checking availability...'
+                    : slugAvailable === true ? 'Available'
+                    : 'Auto-generated from name. Appears in your dashboard URL.'
+                }
+                error={slugAvailable === false ? 'This organization name is already taken. Try a different name.' : undefined}
               >
                 <Input
                   id="onboard-slug"
@@ -81,7 +115,7 @@ export function OnboardingPage() {
                 variant="primary"
                 type="submit"
                 loading={loading}
-                disabled={loading || !name || !slug}
+                disabled={loading || !name || !slug || slugAvailable === false}
                 className={styles.submit}
               >
                 {loading ? 'Creating...' : 'Create workspace'}