Midfielder Maxi Moralez scored a controversial goal six minutes from time to give Velez Sarsfield a 1-0 win over Huracan and the Argentine Clausura championship on Sunday.
Huracan had only needed a draw to win their second championship.
Moralez ran onto a loose ball after Velez substitute Joaquin Larrivey collided with Huracan goalkeeper Gaston Monzon and steered it into the empty net as the referee allowed play to continue.
The scorer, however, was sent off for taking his shirt off during his goal celebration, his second bookable offence of the match.
It was the seventh league title for Velez, world club champions in 1994, who finished two points clear of Huracan in the final standings. They lost only once in 19 matches.
The match was interrupted for half an hour midway through the first half due to a thunderstorm and hail.
Huracan had the ball in the net early on from a header by defender Eduardo Dominguez but a goal was disallowed for offside.
Velez squandered a chance to go in front six minutes after the teams had come back from the rain interruption when Bruno Martinez was brought down by right back Carlos Araujo and they were awarded a penalty but Uruguayan striker Rodrigo Lopez shot weakly and Monzon dived to his right to save.
From the corner Lopez had a header saved on the line by left back Carlos Arano and minutes later Velez defender Sebastian Dominguez headed against the bar.
Huracan were dangerous on the counter-attack and Eduardo Dominguez headed against the Velez bar just before halftime.
Midfielder Matias Defederico finished off a good Huracan move by shooting just wide of the post midway through the second half but Angel Cappa's side showed little of the brilliant passing game that had helped them go eight matches unbeaten and win the last five in a row.
Big guns River Plate and Boca Juniors, who won the previous two titles had a tournament to forget, particularly Boca, who from lifting the Apertura trophy in December plunged into the bottom half of the table in the Clausura.
Boca, however, finished on a winning note on Saturday with a 3-1 win at fourth-placed Colon, having appointed the popular Alfio Basile as coach for a second spell at the club from next season.
Striker Martin Palermo scored twice as Boca fought back from an early deficit, despite having playmaker Juan Roman Riquelme sent off in the 51st minute for dissent.
River were humiliated 2-1 at home by Estudiantes, who fielded a reserve side while their leading players were rested for the first leg of the Libertadores Cup final against Cruzeiro of Brazil in La Plata on Wednesday.
Rosario Central, one of the oldest clubs in the country with four league titles to their name, must fight for their top flight survival in a relegation/promotion playoff against Belgrano of the second-tier National B division.