Elsewhere Atletico Madrid eased to a 3-0 victory over Valencia to pull level with second-place Real Madrid.
Hansi Flick's Barca have now won all five of their opening matches and avenged two humiliating Catalan derby defeats by Girona last season with an emphatic victory at Montilivi.
Yamal's first-half double sent Barcelona on their way with Dani Olmo and Pedri Gonzalez adding further goals after the break.
Barcelona's Lamine Yamal (L) challenges for the ball with Girona's Miguel Gutierrez during the Spanish La Liga football match between Girona and Barcelona at the Montilivi stadium in Girona, Spain, Sept. 15, 2024. Photo by AP |
Cristhian Stuani pulled one back for the hosts, last season's surprise package who finished third in only their fourth season in the top flight.
Girona lost several key players in the summer with coach Michel Sanchez forced to rebuild.
Barcelona have new momentum under former Bayern Munich and Germany coach Flick and produced their best performance of his reign to date.
"I think you can see the confidence we have, and when we have the ball, we have the quality to play and to create chances," Flick told reporters.
"I think today we had a lot of chances to score one or two goals more, but in the end I'm very happy with the performance of the team."
Yamal, 17, and striker Robert Lewandowski came close in the opening exchanges before the teenage star, who led Spain to Euro 2024 victory in the summer, broke the deadlock.
The winger picked the pocket of David Lopez as he tried to bring the ball out of defense, before finishing with aplomb past Paulo Gazzaniga after 30 minutes.
"We started very concentrated, very focused, we pressed them -- we know when we give them the space they need for combinations we get in trouble," said Flick.
"You saw the first goal, it was unbelievably good (pressing) from Lamine."
Yamal grabbed his second before the break with a fine guided finish from the edge of the box after a free-kick was cleared into his path.
Olmo expanded Barcelona's lead early in the second half with a sublime finish from a tight angle, leaving the new arrival from RB Leipzig on three goals from three La Liga matches.
Their fourth came through Pedri when midfielder Marc Casado cleverly played him through and the Canary Islander took his time to sidestep Gazzaniga and finish.
'Turn the page'
Former Espanyol striker Stuani finished from close range with 10 minutes remaining to give Girona a consolation, before Barcelona substitute Ferran Torres was sent off for a high tackle on Yaser Asprilla.
They were the only blots on Barcelona's otherwise pristine copybook as they maintained their 100% record at the start of the campaign.
On Saturday champions Real Madrid earned a 2-0 win at Real Sociedad to move second, four points behind Flick's side.
Girona are seventh after the defeat, with their next match a Champions League debut away at Paris Saint-Germain.
"We have to try and forget about (the loss) as quickly as possible, this week we have a very special game, we have to face that with as much excitement as possible because for us it's an historic match," Stuani told Movistar.
Later on Conor Gallagher and Julian Alvarez netted their first Atletico Madrid goals as Diego Simeone's side earned a comfortable victory over Valencia on Sunday.
Antoine Griezmann tucked home the second for the dominant Rojiblancos at their Metropolitano stadium.
Atletico are unbeaten in their first five fixtures and level on 11 points with Madrid, and Villarreal in fourth.
England international Gallagher, who arrived from Chelsea this summer, tucked home in the 39th minute after Rodrigo de Paul played him in with a clever pass that nutmegged a defender.
"Conor had a great game, he adds a lot of intensity to the match," said Atletico goalkeeper Jan Oblak.
Atletico stayed on top in the second half and Griezmann pounced from close range after 54 minutes, staying alert to react quickest to a loose ball.
Simeone brought summer arrival Julian Alvarez off the bench in the second half and the former Manchester City striker tapped home late on to get his first goal in five appearances for his new club.
"I'm very happy for the win... and for the goal, for the confidence (it brings) and to help the team," Alvarez told Movistar.
"I haven't been feeling the pressure... I'm always ready to play when called on, to help the team."