The Rescue Me star will play a survivor named Randall, according to TV Guide.
The character does not appear in the original Walking Dead comic, created by writer Robert Kirkman.
Zegen's past credits include such films as 2007's The Girl Next Door and 2009's Adventureland. He has also appeared in an episode of The Sopranos and will play a recurring role in the new season of Boardwalk Empire.
Other actors signed up to appear on The Walking Dead include Lauren Cohan, Scott Wilson and Pruitt Taylor Vince.
Meanwhile, Glen Mazzara will assume the role of showrunner, following Frank Darabont's abrupt departure.
The Walking Dead will return to AMC in the US and FX in the UK in October.
> Walking Dead exec: 'New season is more visceral'
Watch a promo for the return of The Walking Dead below: