The pair join lead stars John Barrowman and Eve Myles in the series, about a team of renegade investigators using alien technology to solve crimes.
Gorman (Bleak House) will play Owen Harper, the "raw but charming" medic, while Mori (Absolutely Fabulous) stars as Toshiko Sato, the group's technical expert.
“We were lucky enough to work with Naoko in 2004, on the very first day’s filming of the new Doctor Who, which introduced the character of Toshiko Sato in the episode Aliens of London," said exec producer Russell T Davies. "She was absolutely brilliant, and I knew then that I wanted to bring her back!"
Of Gorman, he added: “Burn was just dazzling in last year’s Bleak House, and attracted our attention immediately. We beat a path to his door! He’s one of the UK’s brightest new talents, and I can’t believe how lucky we are to get him for Torchwood."
The new show begins filming next month for transmission in October on BBC Three and BBC One.