The duo are part of an ensemble cast, which also features Rev's Olivia Colman, This Is England star Vicky McClure, Will Mellor and Jodie Whittaker.
Broadchurch, which is produced by Kudos, will follow a police investigation in a small town which has become the focus of a national media frenzy.
The show is penned by Doctor Who and Torchwood scribe Chris Chibnall.
"Broadchurch focuses on a small British community which finds itself at the eye of a storm. In the wake of one boy's death, the residents of Broadchurch come under scrutiny and suspicion," said Chibnall.
"It's a story of scale and intimacy, as the lives of the characters are laid bare."
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Tennant is cast as DI Alec Hardy, an out-of-town detective, who takes on the case with local girl DS Ellie Miller (Colman), who he beat to a recent promotion.
Darvill, who is leaving his role as Rory Williams in Doctor Who later this year, will play town priest Paul Coates.
The show begins filming in August in Portishead, near Bristol.
"Broadchurch tells the story of a small community and the devastating emotional repercussions for everyone caught up in the mysterious death of a young boy," said ITV drama commissioner Laura Mackie.
"It's not a 'whodunit' or a traditional police drama. Chris Chibnall's superb scripts have attracted a stellar cast."