Lacazette's stunning 36th-minute free-kick put the English club through to a final four meeting with Spanish side Valencia.
"I dont know if it's our best performance this season, but it's good, particularly after beating Watford Monday," said Arsenal coach Unai Emery.
"Our objective was to score, we did that, to win, and to be consistent for 90 minutes.
"I'm very proud of the players, very proud of our work. The players showed personality."
The exit of Carlo Ancelotti's Napoli means no Italian clubs are left in European competition this season.
Unai Emery's side were never troubled by the Italians despite the heated atmosphere in the Stadio San Paolo, even after losing departing midfielder Aaron Ramsey to a hamstring injury after half an hour.
Napoli had turned up the pressure early with a chance to break through after 17 minutes with Kalidou Koulibaly going on the counter-attack, racing up the pitch to cross to Jose Callejon but Petr Cech closed down the Spaniard's weak effort.
Ramsey limped off after 33 minutes with Henrikh Mkhitaryan coming on for the Welsh international.
The Armenian promptly set up Lacazette who was awarded a free-kick after being brought down by Piotr Zielinski.
Lacazette whipped the ball in from 25 metres past a motionless Alex Meret in the Napoli goal.
Emery now faces a reunion with former club Valencia, 5-1 aggregate winners over fellow Spaniards Villarreal, as he targets a fourth Europa League title.
The final will be in Baku on May 29.