Does my divorce decree automatically allow me to change my name after a divorce?

If your divorce decree specifically states that you are able to change your name to a previous name (i.e. birth name or maiden name) then yes, you will be able to use it to change your name. 
If your divorce decree does not state that you are able to change your name, we suggest reaching out to the court that issued your divorce decree to see if you can get your divorce decree amended to include language allowing you to change your name. If you cannot get your divorce decree amended, you will need to undergo a court petitioned name change.
If you wish to have a different last name, you will need to undergo a court petitioned name change.