Jefferson had given Sporting an 87th-minute lead with an 18-yard strike after Artur Moraes had parried into his path from an earlier effort.
A win would have seen the third-placed hosts close to withing four points of the league leaders, but Jardel had other ideas.
The Brazilian found the net with a low drive from the centre of the penalty area four minutes into stoppage time to snare a point in the Lisbon derby.
Martins was sent off for a second bookable offence in the 90th minute, but Belenenses held on for the win.
The strugglers came into the weekend at the foot of the top flight, but came from behind to see off their hosts, who were reduced to 10 men in the second half when Bruno Gallo was shown a second yellow card.
Arouca and Nacional drew 3-3, and Academica played out a 0-0 draw at Vitoria Setubal.
