Microsoft 365 Tenant ID & Domain Lookup
Find the Tenant ID of any Microsoft 365 organization, or check if an onmicrosoft.com subdomain is available — instantly, no login required.
Frequently Asked Questions
How do I find my Microsoft 365 Tenant ID?
Enter your organization’s custom domain (e.g. contoso.com) or onmicrosoft.com subdomain into CheckTenant. The tool queries Microsoft’s public OpenID Connect metadata endpoint and returns the Tenant ID, region, and other details instantly — no login or admin access required.
What is a Microsoft 365 Tenant ID?
A Tenant ID is a globally unique identifier (GUID/UUID) assigned to every Microsoft 365 and Entra ID (Azure AD) organization. It is used in API calls, conditional access policies, cross-tenant collaboration, and identity federation configurations.
How do I check if an onmicrosoft.com domain is available?
Type just the subdomain part (e.g. “contoso”) into CheckTenant’s search box. The tool checks whether contoso.onmicrosoft.com is already registered as a Microsoft 365 tenant. If no tenant is found, the subdomain is available for new tenant creation.
Can I look up a Tenant ID from a custom domain?
Yes. Switch to “Custom Domain” mode and enter any domain (e.g. example.com). If the domain is verified in a Microsoft 365 tenant, CheckTenant returns the Tenant ID, region, organization name, and authentication type.
Is CheckTenant free to use?
Yes, CheckTenant is completely free with no login required. It uses publicly available Microsoft endpoints (OpenID Connect metadata and related APIs) to retrieve tenant information.
Can I reverse lookup a Tenant ID to find its domains?
Yes. Switch to “Tenant ID” mode and paste a GUID. CheckTenant discovers the tenant’s onmicrosoft.com domain, organization name, authentication type, cloud instance, and any custom domains associated with that tenant.