Barkin will play Vera, a legendary real estate broker and Phil's (Ty Burrell) new professional rival. Cannavale will play Louis, the former clown partner of Cam's (Eric Stonestreet) beloved alter-ego, Fizbo.
Both guest appearances are scheduled for early 2012, with Barkin possibly recurring.
Barkin, who made her Broadway debut last season in Larry Kramer's The Normal Heart, took home the Tony Award for Featured Actress in a Play. She was also awarded a 2011 Theatre World Award and Outer Critics Circle Award for her Normal Heart performance. Cannavale, who was nominated for his performance in The Motherf**ker With the Hat, has also been seen on Broadway in Maritius.
"Modern Family" airs Wednesday nights at 9 PM ET on ABC.