Pierre-Emerick Aubameyang scored for the third game in a row while Dembele, Luuk de Jong, and Memphis also got on the scoresheet for Barcelona to help the team climb to fourth place in the standings with 45 points keeping them in the hunt for Champions League qualification.
Former Arsenal striker Aubameyang opened the scoring in the 37th minute.
A neat flick from a corner by Gerard Pique cannoned off the crossbar and was turned in by the striker.
The second goal came in the 73rd with a fantastic finish by substitute Ousmane Dembele.
The French winger received a long pass from Luuk de Jong just outside the box and cleared a defender before firing a rocket of a shot past the goalkeeper into the top left corner of the net.
Dembele then provided assists for the third and fourth goals, in stoppage time.
First, he crossed perfectly to De Jong, who scored with a header, then, in the final play of the game, to Memphis Depay, who stroked in a volley from close range.
Barcelona has not had the best record or luck against Athletic Club and the win will surely boost their confidence as they look forward to a new era after Koeman’s departure.