enfica swept aside Barcelona to heap more pressure on boss Ronald Koeman, whose side have made their worst start to a Champions League campaign.
Barca have lost their opening two matches for the first time, and sit bottom of Group E.
Benfica opened the scoring inside three minutes, when Darwin Nunez cut inside and finished low into the near corner.
Rafa Silva then clipped in a second and Nunez netted a penalty in a comfortable victory for the home side.
Nunez also hit the post from out wide after rounding stranded Barcelona goalkeeper Marc-Andre ter Stegen, who made a rash decision to charge out of his area.
Luuk de Jong missed a glorious chance to level for the visitors early in the first half when presented with a seemingly open goal.
Things got worse for Koeman and the La Liga side when Eric Garcia was sent off for a second booking with three minutes remaining.
Defeat means Barcelona are in danger of failing to reach the knockout stage for the first time since the 2003-04 season, when they did not qualify for the Champions League.
The last time they failed to progress from the first group stage when playing in the competition was in the 2000-01 campaign.