Lamine Yamal stole the show in the first half, netting two quick-fire goals shortly after the half-hour mark to put Hansi Flick's side in control of proceedings.
Dani Olmo's powerful finish added a third goal just after the interval, and Pedri showed fine feet to make it 4-0 to the visitors as the game approached the final quarter.
Barca were slightly fortunate to go in two goals to the good at half time though, as Inigo Martinez's handball which had initially been given as a penalty, was overturned following a VAR review, to the displeasure of the Montilivi crowd.
A late Cristhian Stuani consolation goal and a straight red card for Ferran Torres were not ideal, but did not dampen the spirits as the visitors record an impressive derby day win.
This result sees Barca exact revenge on a Girona side that completed a league double over them last season, winning both meetings 4-2 on their way to finishing third just behind their local rivals.
The resounding win means Flick maintains his 100% start as Barcelona manager, with the club sitting top of the standings after five wins from five.
Conceding so many goals without reply has been costly for Girona, who drop two places to eighth below both Alaves and Espanyol, despite neither side playing today.
GIRONA VS. BARCELONA HIGHLIGHTS
Yamal first goal vs. Girona (30th min, Girona 0-1 Barcelona)
Against players of Yamal's quality, you simply cannot give up possession so easily, and David Lopez was the guilty party here, losing the ball to the teenager on the edge of the area before the attacker fires home beyond Paulo Gazzaniga.
Yamal second goal vs. Girona (37th min, Girona 0-2 Barcelona)
Yamal is at the double! A quick-fire brace for the 17-year-old, who lingers on the egde of the area from a Barca set piece, latching on to the clearance and giving Gazzaniga no chance with a superb low finish into the corner.
Dani Olmo goal vs. Girona (47th min, Girona 0-3 Barcelona)
A shaky start to the second half by Girona has been punished and Barcelona are now surely out of sight!
After losing possession in the first few seconds of the half, Girona give the ball away in a dangerous area again just moments later, and Olmo is there to pounce with a devastating finish from an acute angle beyond a stricken Gazzaniga.
Pedri goal vs. Girona (64th min, Girona 0-4 Barcelona)
With Girona now looking leggy and demoralised, Barcelona knock the ball around with confidence, and a stunning through ball from Marc Casado splits open the home defence before Pedri rounds the goalkeeper and fires home a fourth goal.
Stuani goal vs. Barcelona (80th min, Girona 1-4 Barcelona)
With Barca coasting and victory already secured, a slight bit of complacency creeps in as Stuani nets a consolation goal for the hosts.
Fine work from Portu on the right sees the winger go around Marc-Andre ter Stegen, and his square ball finds the veteran striker who had the simple task of diverting the ball into an empty net.
MAN OF THE MATCH - LAMINE YAMAL
There were many candidates for this award, but it was Yamal who put Barcelona in such a commanding position in the first half with two very different goals.
The first showed his grit and determination, winning back possession pressing from the front before staying composed to finish one-on-one, while the second was a superb curling effort from the egde of the area as the teenage sensation as set to make all the headlines across Spain tonight.
GIRONA VS. BARCELONA MATCH STATS
Possession: Girona 47%-53% Barcelona
Shots: Girona 5-17 Barcelona
Shots on target: Girona 4-8 Barcelona
Corners: Girona 1-6 Barcelona
Fouls: Girona 9-3 Barcelona
BEST STATS
WHAT NEXT?
Both clubs are in Champions League action this midweek, with Girona making their debut in the competition away to Paris Saint-Germain, before going to Valencia in La Liga next weekend.
Barcelona make the short trip home to welcome Monaco in Europe on Thursday, and they will then face Villarreal on the road when they return to domestic matters in a weeks' time.