On the series Martin will play Scott Welker, an artistic director of a theatre company who has a history with a couple of the characters. His character is also reported to be a possible love interest for Julia (Debra Messing).
Martin, who has been seen on NBC's "Law & Order," has also appeared on Broadway in Timon of Athens, The Government Inspector and The Merchant of Venice.
He will join previously announced Season Two additions Academy Award winner Jennifer Hudson, Tony Award nominees Jeremy Jordan and Sean Hayes, and Andy Mientus of Off-Broadway's Carrie.
For the series' second season, Josh Safran succeeds Theresa Rebeck (Seminar) as showrunner — the person in charge of all aspects of the story.