TV Line reports that the Rescue Me actor will play famed gangster Benjamin 'Bugsy' Siegel in the upcoming second season.
The character will be introduced as a tough street kid and aspiring gangster known as Benny, since his nickname did not enter common use until the late 1930s.
Zegen played Damien Keefe on Rescue Me from 2004 to 2010, and has also featured in episodes of The Sopranos and short-lived NBC medical drama Mercy. His film credits include roles in 2007 drama The Girl Next Door and 2009's Adventureland.
The second season of Boardwalk Empire will also introduce Ralph and Frank, the young brothers of Al Capone (Stephen Graham). Both roles are currently yet to be cast.
HBO renewed the series in September, after just one episode had aired.