The pair will take part in the sixth series of the Channel 4 reality show, which will see celebrities take part in SAS winter warfare training in the south islands of New Zealand.
Contestants are not eliminated and are instead culled by the directing staff or able to withdraw themselves, with those who remain at the end declared the winners.
Alongside Torchwood actor Barrowman and Johnson, who is the sister of former UK prime minister Boris Johnson, reality star Pete Wicks, who previously competed on the 2022 edition of the show, will appear on the show at the same time as he appears on the BBC’s Strictly Come Dancing.
Wicks was forced to withdraw during his first appearance on the show after breaking his ribs in the second episode of series four, which took place in Jordan.
Speaking about his return to the show, Wicks said: ““After being medically withdrawn in Jordan, I felt like I hadn’t given it my best, and that bothered me.
“So when I got an opportunity to go back, I jumped in, and I would do it again.”
Johnson added: “This experience was much more than I was expecting. I mean, given how ghastly reality television can be, this was top notch. Really top notch.”
Reality TV Star Bianca Gascoigne, former England rugby captain Chris Robshaw, TV Presenter Cherry Healey, former professional boxer Anthony Ogogo, comedian Shazia Mirza, former Love Island contestant Georgia Harrison, reality star Bobby Norris, and influencer Marnie Simpson will also join them on the show.
The line-up will be completed by former Olympic gymnast Ellie Downie, comedian Tez Ilyas, professional basketball player Ovie Soko, and World Heavyweight Champion boxer Lani Daniels.
Chief instructor Billy Billingham will also return with his team of directing staff Foxy (Jason Fox), Rudy Reyes and Chris Oliver, while they will be joined by former Special Forces psychologist Dr Alia Bojilova.
The show will see contestants exposed to constant cold and wet as they undertake an array of SAS training exercises.
Speaking of the new series, Billingham said: “As our course progresses, the standards expected will never faulter.
“From the very first second, the recruits will be expected to give 100%.
“Anyone not doing so will be gone before they have time to make their excuses. Our time is invaluable – we will not waste it on anyone not worthy.”
Series six of Celebrity SAS: Who Dares Wins will air on Channel 4 every Sunday and Monday from September 22, at 9pm.