On 20 May 1966, Peñarol won their third Copa Libertadores, beating River Plate with two extra-time goals in a playoff.
The Uruguayans were the most experienced side in the tournament's history up to that point, having appeared in three previous finals (as winners in 1960 and 1961 and as runners-up in 1963), while River Plate were playing in their first one. But the Argentine side reached it by beating the 1965 champions, Independiente, in the semifinal group stage.
The final was scheduled for two legs, with the winner decided on points. Peñarol won the first leg in Montevideo on 12 May by the score of 2-0, but River Plate rallied six days later to win 3-2 in Buenos Aires with a 69th-minute goal from midfielder Ermindo Onega to force a playoff match.
Playing before a crowd of 40,240 in Santiago's Estadio Nacional, Onega struck twice in the first half (29', 42') to lift the Argentinians to a 2-0 lead. But Peñarol narrowed the margin in the 65th minute with a goal from striker Alberto Spencer, then drew level six minutes later with a River Plate own goal.
Still 2-2 at the end of regulation, the match went to extra time, where Peñarol dominated. Spencer scored in the 102nd minute to take the lead, then forward Pedro Rocha found the back of the net in the 109th minute to cap the 4-2 comeback.