Sporting seemed to be set for a win when Jefferson fired in a loose ball with three minutes to play.
But Benfica, who had taken 80 minutes to force their first corner, stunned the Alvalade when defender Jardel fired home in stoppage time after a goalmouth scramble.
The first 85 minutes were mainly an ugly scrap with neither team prepared to give an inch or capable of creating chances.
Champions Benfica, who have 50 points from 20 games, had their lead at the top cut to four points by Porto who won 2-0 at Moreirense on Saturday with goals in each half from Jackson Martinez and Casemiro.
Sporting are third on 43 points.
Gil Vicente won their first away match of the season when they beat Maritimo 2-1, a result which also moved them off the bottom of the table where they were replaced by Penafiel following a 2-1 defeat at Pacos de Ferreira.
Venezuelan forward Mario Rondon scored a hat-trick for Nacional in their 3-3 draw away to Arouca.
Nacional raced to a 2-0 lead inside the first half hour, Arouca hit back to lead 3-2 with five minutes left before Rondon grabbed an equaliser.
(Writing by Brian Homewood; Editing by Ed Osmond)
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