The Love and Other Drugs star will play a character named Kenny in the ABC comedy's third run, according to Entertainment Weekly.
Living next door to the Dunphys, Kenny idolises Phil (Ty Burrell) and begins to mimic his neighbour.
Gad previously starred alongside Modern Family's Ty Burrell in Fox sitcom Back to You and recently appeared in South Park creators Trey Parker and Matt Stone's Broadway musical comedy The Book of Mormon.
He has also appeared in episodes of Bored to Death, Numb3rs and Party Down, and will play a voice role in Adam Brody's new MTV animated series Good Vibes.
Other stars confirmed to appear in future episodes of Modern Family include Tim Blake Nelson and the returning Benjamin Bratt.
The third season begins on Wednesday, September 21 on ABC.
> Sofia Vergara: 'Modern Family cast are like their characters'
Watch a promo for the third season of Modern Family below: