The British actor is the latest addition to the Ryan Reynolds-Hugh Jackman team-up.
Matthew Macfadyen has his first post-Succession gig lined up.
The actor will join the cast of Deadpool 3, ET can confirm, though details on who Macfadyen is playing are being kept under wraps.
He is the latest addition to the upcoming threequel, which returns Ryan Reynolds as Wade Wilson/Deadpool. Hugh Jackman also reprises his role as Logan/Wolverine, with The Crown's Emma Corrin also on board.
Macfadyen won a supporting actor Emmy and BAFTA Award in 2022 for his role as Tom Wambsgans, Shiv Roy's husband, on the HBO family drama, which recently kicked off its fourth and final season.
Deadline first reported the news.
Following his Emmy win in September, the actor spoke to ET about being "shell-shocked" that he came out victorious.
"You never expect it'll be you and it's nice. It's a really nice thing. It's very flattering," the British star said, adding he had family on his mind moments before the envelope was opened and revealed his name.
"I was thinking of my kids watching on, family," he said. "And that's when she reads my name out, so that was that. That's what was going through my head, was my children."
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