The Campos Racing driver started on reverse-grid pole for Saturday’s race as the series made its first visit to the principality since its GP3 guise in 2012.
Despite one safety car, it was a sedate race for F3, with few moves among the top 10.
After the restart, Marti managed to build a convincing gap, having opened up three seconds to Leonardo Fornaroli in the first 11 laps of racing before crossing the line eight seconds ahead of the Trident driver.
Gregoire Saucy finished third for ART Grand Prix to take his third podium of the season, sitting second in the drivers’ standings.
With Marti leading off the line ahead of Fornaroli, the only early move at the front came courtesy of Saucy passing Jenzer’s Taylor Barnard for third around the outside of Turn 1.
Further back, Hitech driver Luke Browning made it past Caio Collet (MP) and Sunday polesitter Gabriele Mini clinched 11th from Prema rival Dino Beganovic.
Source: Autosport