Juan Quintero's stunning extra-time strike helped River Plate come from a goal down to record a historic victory over 10-man Boca Juniors in Madrid and win the Copa Libertadores.
Quintero scored in the 109th minute of the second leg before team-mate Gonzalo Martinez confirmed a 3-1 victory on the night for River - and a 5-3 win on aggregate.
Martinez ran the ball into an empty net with the last kick of the game after Boca keeper Esteban Andrada had been caught upfield at a corner.
That ended a dramatic extra-time period, which had started with the sending off of Boca midfielder Wilmar Barrios for a second bookable offence - a two-footed challenge on Exequiel Palacios.
Martinez's goal completed a dramatic turnaround in a match moved more than 6,000 miles from Buenos Aires, in Argentina, to the Spanish capital after its postponement in November, when Boca's team bus was attacked by River fans, causing injury to players and staff.
Dario Benedetto had fired Boca into the lead shortly before half-time at Real Madrid's Bernabeu Stadium.
Benedetto finished off a devastating counter-attack and celebrated by sticking his tongue out at River defender Gonzalo Montiel.
Former Barcelona striker Lucas Pratto equalised for River in the second half when he slotted home a cut-back at the end of a slick passing move.
The atmosphere was raucous as the crowd spent the majority of the match standing up - and many River fans were seen crying with delight when their team equalised. Those supporters were delirious with joy as their team came back to win in the additional half-an-hour.
It was the first time in the 58-year history of the Copa Libertadores that the two great Buenos Aires rivals have faced each other in the two-legged final, with the first leg, on 11 November, ending in a 2-2 draw.
The build-up to this hugely anticipated fixture was full of controversy after the second leg - originally scheduled for 24 November - was postponed as a result of the bus attack.
Boca players suffered cuts from broken windows when their team bus was attacked by River fans and were also affected by the tear gas used by police to disperse the crowds.
A number of players, including Boca's Carlos Tevez - the former Manchester City, Manchester United and Juventus striker - reportedly suffered from dizziness and vomiting.
Boca argued River should have been disqualified from the competition, but a late attempt to postpone Sunday's rescheduled second leg match was rejected by the Court of Arbitration for Sport (Cas).
The Copa Libertadores is the showpiece club competition in South America, equivalent to the Champions League in Europe, and there were several household names in attendance at the Bernabeu, including Barcelona's Lionel Messi and Juventus' Pablo Dybala.