Jon Hamm has joined the cast of Amazon Prime's upcoming series Good Omens, the streaming service revealed Monday.

The Emmy-winning Mad Men star will join Michael Sheen and David Tennant in the six-part fantasy/horror/comedy series, which is being adapted by Neil Gaiman from his and the late Terry Pratchett's collaborative novel of the same name.

Hamm will play the archangel Gabriel, God's primary messenger. Gabriel had a small part in the novel, and Gaiman explained why he's expanding the role in a statement:

"Once we had finished writing Good Omens, back in the dawn of prehistory, Terry Pratchett and I started plotting a sequel," Gaiman said. "There would have been a lot of angels in the sequel. When Good Omens was first published and was snapped up for the first time by Hollywood, Terry and I took joy in introducing our angels into the plot of a movie that was never made. So when, almost thirty years later, I started writing Good Omens for TV, one thing I knew was that our angels would have to be in there.

"The leader of these angels is Gabriel," he continued. "He is everything that Aziraphale isn't: he's tall, good-looking, charismatic and impeccably dressed. We were fortunate that Jon Hamm was available, given that he is already all of these things without even having to act. We were even more fortunate that he's a fan of the books and a remarkable actor."

David Tennant Is the Demon to Michael Sheen's Angel in New Amazon Series

Good Omens the series is set in 2018 as the End Times near and follows an angel named Aziraphale (Sheen) and a demon named Crowley (Tennant), Heaven and Hell's representatives on Earth. They don't really want to give up their comfortable earthly existences, so they team up to keep an eye on the child destined to be the Antichrist. But there's a mix-up on which kid the Antichrist actually is.

Since Mad Men, Hamm has split his time between movies and TV. On the film side, he's starred in Baby Driver and Keeping Up with the Joneses. His recent TV credits include Unbreakable Kimmy Schmidt, Wet Hot American Summer: First Day of Camp and The Last Man on Earth.

Good Omens will stream on Amazon Prime in 2019, followed by a broadcast run on BBC Two in the UK.