The former Leeds stopper had two successful loan spells with Rovers last season and starred in the club's Johnstone's Paint Trophy final win over Bristol Rovers.
Sullivan, who has signed a two-year deal, said: "I enjoyed my time here in my two spells on loan and barring any problems I'll be a Doncaster Rovers player, which is something that I am really looking forward to."
Rovers chairman John Ryan said: "It's a great coup for Doncaster Rovers and I feel it shows our intentions for next season, as we bring in our third quality player to assemble a squad that will get us in the Championship.
"He almost single-handedly won the Johnstone's Paint Trophy, with his incredible saves. Hopefully that trophy will be the first of many he wins here."