Barcelona needed only three minutes to take the lead. Raphinha produced a brilliant individual move, leaving two Feyenoord defenders behind before cutting inside and firing a left-footed finish beyond the goalkeeper.
The hosts continued to dominate and doubled their advantage in the 22nd minute. Karim Adeyemi collected the ball on the right before cutting inside and curling a superb effort into the far corner to make it 2–0.
Feyenoord struggled to cope with Barcelona's intensity and pressing, while Hansi Flick's side continued to create openings around the visitors' penalty area.
Raphinha struck again in the 57th minute to put Barcelona firmly in control. The Brazilian finished a defence-splitting pass from Pedri, taking his tally to two goals for the night.
Lamine Yamal then added a fourth in the 77th minute with a clever direct free-kick, firing the ball through a gap in the Feyenoord wall.
The Dutch side pulled one goal back five minutes later when Sem Steijn found the net in the 82nd minute, but Barcelona quickly restored their four-goal advantage.
Substitute Gabriel Jesus completed the rout in the 85th minute, scoring his first goal for Barcelona with a backward header after another contribution from Yamal.
• Barcelona: Raphinha 3', 57', Karim Adeyemi 22', Lamine Yamal 77', Gabriel Jesus 85'
• Feyenoord: Sem Steijn 82'
The result gives Barcelona an ideal start to their Champions League campaign and underlines the attacking strength of Flick's side. Raphinha, who was appointed captain by Flick, continues to enjoy an outstanding start to the season, while Yamal also played a decisive role with a goal and two assists.
For Gabriel Jesus, the goal was particularly significant as it marked his first for Barcelona. The Brazilian striker came off the bench and needed only one touch to make his mark on the Champions League opener.
Barcelona will now return to domestic action before their next Champions League fixture, with the Catalan giants travelling to face Levante in La Liga on Sunday.