The Union signed 18-year-old Cristhian Hernandez as the team's third "homegrown player" to come from its youth academy.
Hernandez will occupy an international roster spot and is eligible to play in the season opener against the Portland Timbers on March 12. Terms were not disclosed.
Hernandez has played in reserve league matches for the Union and scored in the game-winner in a friendly against Everton last July.
He attended St. Benedict's Prep in New Jersey, where he helped lead the team to a 24-0 record, a No. 1 ranking and the school's seventh national championship as a senior. He was ranked among the top 10 high school seniors in the country by ESPN.
He joins Jimmy McLaughlin and Zach Pfeffer as players to come out of the Union Academy.