Dembele latched on to a through ball from Messi and knocked it through the legs of Sevilla keeper Bono to give Barca a deserved lead in the 29th minute after dominating the first half and barely giving their hosts a hint of a chance.
Sevilla improved after the break but Barca continued to have the edge and found a second goal five minutes from time when Messi played a one-two with teenage midfielder Ilaix Moriba and netted with a scrappy finish after having his first effort saved by Bono.
The result ended a six-game winning streak in the league for Sevilla, who remained fourth in the standings on 48 points, while Barca climbed above Real Madrid into second place with 53 points, two behind Atletico but having played two more games.
Sevilla can avenge the defeat on Wednesday…