The project will star Sarah Michelle Gellar as identical twin Bridget, who assumes her dead sister Siobhan's identity to hide from the mob.
Deadline reports that Fantastic Four star Gruffudd will play Andrew, Siobhan's sophisticated, wealthy husband who begins to notice changes in his wife.
He first rose to fame in the Hornblower series of television films and has recently performed voice work in episodes of Family Guy and American Dad.
Salt actor Mike Colter has also signed up to play Bridget's AA sponsor, having recently appeared in episodes of Law & Order: SVU, Royal Pains and The Good Wife.
Former Lost star Nestor Carbonell will also appear in the pilot as an FBI agent assigned to protect Bridget.
Ringer is being written by Eric Carmelo and Nicole Snyder and will air its pilot episode later this year.