Martinez was absent from the Atlanta side for their match against Inter Miami today ahead of his big money move.
River will receive $324,000 and his original club Huracán over half a million in solidarity payments.
One of Atlanta’s ‘Designated Players’, Pity has made 54 appearances with a return of 11 goals and 14 assists.
He joined the MLS side after playing a key role in River Plate’s famous 2018 Copa Libertadores win over Boca Juniors.
Martinez scored the third and final goal in the second leg, making him a hero among the River fans.