The Ecuadorian-American who has represented both countries at the youth international level celebrated his 18th birthday by signing his first professional contract after spending the last two years in the Union Academy.
Zambrano-Delgado led the Union U17s to the MLS Next Cup title last year with a pair of goals in the final and then also scored the game-winning goal in the first ever MLS Next All-Star Game.
He becomes the second player to leave the Union Academy to sign with Benfica. Philadelphia native CJ dos Santos signed with Benfica as a 16-year-old from the Union in 2016 and the goalkeeper featured with Benfica B before returning to the U.S. to sign with Inter Miami last year.
Though the Union never signed Zambrano-Delgado to a professional contract, they will receive training compensation under FIFA regulations.