The 21-year-old Sosa represented Argentina at the 2019 FIFA U-20 World Cup and could be in line to play for his home country in the Tokyo Olympics. Atlanta United announced the deal Friday.
Sosa has spent his entire professional career with River Plate, earning a promotion to the senior roster during the 2018-19 season. He made 14 appearances in 2020, including six games in the Copa Libertadores and six more in the Copa Diego Maradona.
The acquisition of Sosa figures to be a big part of United's efforts to bounce back under new coach Gabriel Heinze after missing the playoffs for the first time in franchise history in 2020. In addition to its league schedule, the team is set to make its third straight appearance in the CONCACF Champions League. United will face Costa Rican club Alajuelense in the opening round April 7 and 14.
"Santiago is a holding midfielder who understands and reads the game exceptionally well for someone his age,'' Atlanta technical director Carlos Bocanegra said in a statement. "He broke through into the first team at River last year, experiencing numerous big matches in both Copa Libertadores and the Copa Diego Maradona, in addition to his time with Argentina U-20s.''