Jorge Jesus' side capitalised on Sporting Lisbon's draw at Estoril on Saturday with a goal in either half as the visitors, who finished with 10 men, lost for just the second time in the league this season.
The clash was the first outing since the death of former Benfica and Portugal striker Eusebio, and all the home players wore the legendary striker's name on the back of their shirts in remembrance of the man who scored 733 goals in 745 matches during a remarkable career.
Goals from Rodrigo and Ezequiel Garay sealed victory over Porto for the first time at home since the 2009-10 season.
Rodrigo opened the scoring in the 13th minute as he latched onto a perfectly weighted throughball and lashed it into the roof of Helton's net.
Garay added the second moments after Benfica had been denied a penalty for a handball by Eliaquim Mangala, as the central defender rose to meet a 53rd-minute corner and score his third league goal of the season.
The visitors' misery was added to with 15 minutes to play as Danilo was dismissed after collecting a second yellow card.
Olhanense ended a run of five consecutive league defeats with a 2-1 victory over Vitoria Setubal, a win that moves them off the bottom of the table.
After going behind to a Ramon Cardozo goal in the ninth minute Olhanense fought back and secured their third league win thanks to goals from Agon Mehmeti and Federico Dionisi.
It was also a good day for Arouca as they beat Belenenses 2-0 to record back-to-back victories, but Pacos de Ferreira were the big losers in the relegation battle as they fell to a 4-2 defeat against Academica.
Pacos had been 2-1 ahead thanks to Manuel Jose's penalty and Bebe's strike before the interval, but they conceded three second half goals and had Filipe Anunciacao sent off as they slipped to the foot of the table.
In Sunday's final game Nacional moved into fifth place with a 2-0 win over mid-table Gil Vicente, who suffered a fourth consecutive league defeat.
Brazilian defender Fernando Marcal gave the hosts the lead in the 36th minute with his first goal of the season, and his compatriot Diego Barcellos confirmed the victory just after the hour.
