Boca Juniors won the Argentine Apertura championship on Tuesday, finishing top of a three-way play-off by the slimmest of margins after losing 1-0 to rank outsiders Tigre in the final match.
Boca, Tigre and San Lorenzo all finished the mini-league with three points from two games but Boca took the title on goal difference.
Boca had a goal difference of one, compared to zero for Tigre and minus one for San Lorenzo.
The play-off was held after the three teams finished level on points after the regular championship, where goal difference is not used to decide the title.
It was Boca's 23rd title under the various formats used since Argentine football turned professional in 1931 but they are still 10 titles behind their arch-rivals River.
It was their first title in five championships, considered an unusually long time for a club who have traditionally dominated the domestic game alongside River.
Their last success was the 2005/06 season's Clausura championship.
Two league championships, the Apertura and Clausura, are played per season in Argentina, the 20 teams meeting each other once in each.
Boca, missing playmaker Juan Roman Riquelme through suspension, had an early chance when Luciano Figueroa's shot was saved by Luis Ardente.
They wasted an even better opening when Jesus Datolo broke clear of the Tigre defence but failed to control the ball.
Martin Morel, the championship's topscorer with 13 goals, had Tigre's best first-half effort with a low shot from 20 metres which was comfortably held by Boca goalkeeper Javier Garcia.
Tigre rarely looked menacing but went ahead in the 68th minute when Garcia misjudged Matias Gimenez's long cross from the left and Leandro Lazzaro headed in.
The tearful Garcia, whose blunder also gave away a goal in the 3-1 win over San Lorenzo on Saturday, was immediately substituted by Josue Ayala.
Another goal would have handed Tigre their first title but Boca, despite looking jittery at the back and having Rodrigo Palacio sent off in injury-time when he was booked for petulantly kicking the ball away and then shown another yellow card for protesting the first one, held on.