The actor will appear in an upcoming episode of the show as a friend of a friend of Cameron (Eric Stonestreet) and Mitchell (Jesse Tyler Ferguson), TV Line reports.
Cameron and Mitchell assume that Marini's character Julian is gay.
However, they end up finding out that he is straight when they all go on a night out with Claire (Julie Bowen).
Marini is probably best known for his role as Luc in Brothers & Sisters. He has also guest starred in shows including Ugly Betty, Dirty Sexy Money, Nip/Tuck, Castle and, most recently, Switched At Birth.
Modern Family, which recently won a number of Emmy Awards, will begin its third season tonight at 9/8c on ABC.
> Jesse Tyler Ferguson teases Modern Family baby plot
Watch a trailer for the third season of Modern Family below: