Can I undo a “mark all as read” operation in Gmail?

Question

I accidentally marked all of my messages as read in Gmail.

Can I undo this action?

Answer 1

As Alex has said in his comment, "If you haven’t touched anything after, you should have the option to undo the previous action in the notification that appears at the top of Gmail." but if you have and that notification bar no longer exists, then I do not think so that there is absolutely any way to undo it.

If you use GMail Offline Feature, then you can see what messages you haven't still read but that too you would have to do before you sync.