River, record 33 times league champions and never relegated, will meet Nacional B (second division) side Belgrano over two legs in the relegation-promotion play-offs after substitute Leandro Diaz scored a stoppage-time winner at a packed Monumental.
Striker Silvio Romero put Lanus, who finished the Clausura championship second to champions Velez Sarsfield, ahead on the half hour with a low shot into the bottom corner from the edge of the box.
River equalised two minutes after half-time when teenage midfielder Erik Lamela turned in right-back Paulo Ferrari's cross, sparking a frenetic series of attacks by the home team.
Lamela and striker Leandro Caruso missed chances to win the match for River before Diaz's sucker punch, a superb shot inside the far post beyond diving Argentina goalkeeper Juan Pablo Carrizo.
Gimnasia, who had gone two goals up, were held 2-2 at home in La Plata by Boca Juniors after conceding a late equaliser by defender Cristian Cellay from a header by Martin Palermo in the striker's last match before retiring.
