This article will show you how to find your primary (or default) domain name in Office 365.
Find the Default Office 365 Domain Name Using the Web Console
To find your default Office 365 domain name using the web console do the following:
- Log in to your admin console
- Expand Settings and click on Domain
- Identify the domain which is suffixed with (Default)

Find the Default Office 365 Domain Name Using PowerShell
To find your default Office 365 domain name using PowerShell do the following:
- Log in to MS Online Services
- Use the Get-MsolDomain command
The code could e.g. look like this:
Connect-MsolService
(Get-MsolDomain | Where-Object {$_.isDefault}).name
(thanks to Matthew, Enterprise Engineer at AllCovered)
If you’re looking for your Office 365 tenant name (which differs from your default Office 365 domain name) have a look at this article.
If you’re still using a mix of AD Users & Computers, Azure AD web console, Exchange Admin Center and on-premises Exchange Admin Console consider this: