Raphinha continued his strong start to the season by opening the scoring in the 37th minute, while Fermin Lopez added a late second to secure another comfortable win for Hansi Flick's side.
Barcelona dominated possession and created the better chances, although Athletic threatened at times and had opportunities to make the contest closer.
The hosts went close several times before breaking the deadlock. Lamine Yamal tested Unai Simon with a curling effort, while Raphinha headed against the post with the Athletic goalkeeper beaten.
Pedri appeared to have put Barcelona ahead before the half-hour mark, but the goal was ruled out for offside earlier in the move.
Barcelona eventually found the breakthrough when Pedri released Raphinha behind the Athletic defence. The Brazilian rounded Simon before calmly finishing into an empty net.
Raphinha almost doubled his tally before half-time, only for Simon to produce an excellent save to deny Yamal.
Athletic continued to struggle to contain Barcelona after the break. Anthony Gordon missed a good opportunity early in the second half, firing over from a promising position, before Yamal struck the crossbar.
Athletic became more dangerous as the match progressed and Oihan Sancet went close when his low effort struck the post.
Rodri was then introduced to a huge reception from the home supporters as the former Manchester City midfielder made his Barcelona debut.
Barcelona eventually sealed the points with less than 10 minutes remaining. Fermin pounced after Aymeric Laporte's mistake to score the second goal and cap an impressive individual performance.
Flick was pleased with Barcelona's attacking display but admitted his team could have been more composed after taking the lead.
"I don't know if Raphinha will be our No. 9," Flick told DAZN. "For me, it's important that the plan works out, whether Raphinha is the number nine or not.
"Because of his dynamism, his runs off the ball, and his intensity, he's the right one."
Raphinha has now scored three goals in Barcelona's opening two league matches, underlining his importance to Flick's attack.
The German coach was also impressed by youngster Xavi Espart, who produced another strong performance on the left and wore his new No. 12 shirt after being promoted to the first team.
"Espart has great technique, he handles situations very well, both in attack and defence," Flick said.
"I was surprised by his performance today on the left wing. He's top-notch. He has room for improvement, like everyone, but he's a fantastic player."
The victory leaves Barcelona top of La Liga after two matches, while Athletic have suffered another away defeat and sit second from bottom.