King took to Twitter to share the news, writing, "It's official: The man in black fled across the desert and the gunslinger followed. #DarkTowerMovie @McConaughey @IdrisElba."
Elba will play Roland Deschain, the Gunslinger, while McConaughey will play villain Randall Flagg, or the Man in Black.
Elba further confirmed the news on Twitter, mentioning his famous co-star. "You have one new follower. #DarkTowerMovie," he wrote.
"Come and get me, I look forward to it," McConaughey replied on the social media site.
It is uncertain which books from the Dark Tower series the upcoming film will focus on, but co-writer and director Nikolaj Arcel said the story will "take place in our day, in the modern world," according to Entertainment Weekly.
King also chimed in, telling ET "[The movie] starts in media res, in the middle of the story instead of at the beginning, which may upset some of the fans a little bit, but they'll get behind it, because it is the story."
"The thing is, it's been a looong trip from the books to the film," King said.
"When you think about it, I started these stories as a senior in college, sitting in a little [expletive] cabin beside the river in Maine, and finally this thing is actually in pre-production now. I'm delighted, and I'm a little bit surprised."