Gmail – How to Undo ‘Mark All as Read’ Operation

There are two ways:

1. Perform a search to find all the messages you want to select. (You can use is:unread too)
2. Click select all
3. You will see that above the first item in the list there is a banner that says something like: All 50 conversations on this page are selected. Select all # conversations in "..."; click that link.

You can also do this with a filter, by creating the filter, choosing to mark the messages as read, and then choosing to apply the filter to the matching conversations. Then simply delete the filter.

Search for string before:date (date in format yyyy-mm-dd) and then choose More > Mark all as read.

Watch out: This will select also emails from this date! I.e. before:2012-07-12 will select email received before 12 July 2012, but also emails received on 12 July 2012.