If you have just a handful of unread messages in Gmail, it’s easy to select them & mark as read. But what if you have hundreds or even thousands of unread messages? This article describes a way to mark them all read in one go.
Start by opening Settings & head on over to the Filters tab:
Scroll to the end & click Create. Enter * in the From field & click Create again:
Click OK to Confirm:
Select “Mark as Read” & “Apply Filter to Matching Conversations” & click Create:
Wait a minute for the confirmation message that says “Your filter was created.” It might take a while depending on how many messages Gmail has to mark as read. Once you see the confirmation, all your messages have been marked as read. Now delete that filter. Otherwise, all new incoming mail will also get marked as read automatically.