Tate will reprise her role as assistant Donna Noble for one of the four specials planned for next year, while Life On Mars star Simm will return as villainous Time Lord The Master.
A Who source told The Sun: "Fans will be delighted to see Catherine back. She was one of the wackiest companions of all time and she brought heaps of humour. And it's great to have John back - he and David Tennant have a real nemesis chemistry."
Veteran actor Bernard Cribbins will also be back to play Donna's grandfather Wilf.
Tate, whose character's memory was wiped in last year's series finale, earlier expressed her desire to return to the cult show, saying: "In science fiction, anything is possible."