The British stage, television and film actor spoke about teaming up with Olivia Colman, who is taking on the role of the Queen.
"I'm thrilled to be joining the new cast of The Crown and to be working with Olivia Colman again," he said in a statement. "I look forward to becoming her 'liege man of life and limb.'"
Actors Claire Foy, 34, and Matt Smith, 35, played a young Elizabeth and Philip in the show's first two seasons, alongside Vanessa Kirby as Princess Margaret and Victoria Hamilton as the Queen Mother.
Colman, 44, best known for her roles in The Night Manager, Fleabag, Broadchurch, and Peep Show, previously praised Foy as "an absolute genius".
"I was utterly gripped watching (The Crown). A proper 'just one more' feeling. I think Claire Foy is an absolute genius," she said. "She's an incredibly hard act to follow. I'm basically going to rewatch every episode and copy her!"