A goal from Pablo Hernandez in the 86th minute allowed Argentinos Juniors to secure a 1-0 win over newly-crowned champions Newell's on Sunday, keeping Arg. Juniors in the top flight.
San Martin's 3-1 defeat at River Plate means that it will be San Martin who is headed down to the second division along with Union and Independiente.
Arg. Juniors was in line to take San Martin's place in the relegation fight, but Hernandez saved his side with a headed goal from a corner kick four minutes from time to pull his club out of the fire.
Union is headed down after a 3-0 away defeat to Racing, while Independiente was held to a 1-1 draw at Colon as the two sides traded first-half goals.
River Plate's 3-1 home victory over San Martin clinched second place for the club, while Lanus finished third after being held to a 2-2 draw at San Lorenzo, which scored twice in the second half to overturn a 2-0 halftime deficit.
Quilmes closed out its season with a 3-2 win over Tigre as Miguel Caneo netted the winning goal in the 52nd minute, Estudiantes and Belgrano played to a 1-1 draw, Ezequiel Rescaldani tallied both goals for Velez Sarsfield in a 2-1 win over Atletico Rafaela, Arsenal secured a 4-0 triumph over All Boys and Boca Juniors closed out a disappointing campaign with a 1-1 draw at Godoy Cruz.