It’s pretty easy now to get your Office 365 tenant ID. Here, I will demonstrate three ways, by which you can retrieve your tenant ID :-

• Azure portal (new and old)
• PowerShell
• SharePoint Online

Azure Portal

New

This, by far is the best and the easiest way to retrieve the tenant ID. In here,

• Login to Azure portal as administrator.
• Select Azure Active Directory.
• Select the directory that you’re using with your Office 365 tenant.
• Under Proprties, you can find the Directory ID. This is your tenant ID.

  

Old

This option isn’t bad either. However, you won’t get a proper interface like the new Azure portal. In here,

• Login to Azure portal as administrator.
• Click, Active Directory.
• From the list, select the directory that you’re using with your Office 365 tenant.
• The tenant id will be displayed as part of your URL. You’ll have to manually extract it from there!

  https://manage.windowsazure.com/tenantname#Workspaces/ActiveDirectoryExtension/Directory/<TenantID>/directoryQuickStart

PowerShell

If you have, SharePoint Online Management Shell installed on your computer then, you can get the same by running this PowerShell command

#login with your Office 365 admin id
Login-AzureRmAccount

You will be prompted for userID/password.

Once authenticated, your tenant ID will be displayed in the console.

SharePoint Online

This is good if, you’re inside your SharePoint Online environment. Instead of going to Azure or PowerShell, you can directly extract the tenant ID from here as well.

Go to Site Settings -> Site App Permissions.

There, you’ll find, App Identifier for each installed app. In this identifier, for any of the installed app, the part after the symbol, ‘@‘ is your tenant ID.

You can directly reach this page by hitting the following URL. Again, it’s just a direct shortcut of the navigation provided above,
https://<sharePointUrl>/_layouts/appprincipals.aspx