The Campos Racing driver started on reverse-grid pole for Saturday's race as the series made its first visit to the principality since GP3 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.
The safety car was quickly deployed after MP Motorsport driver Jonny Edgar found the wall at Turn 1, ending his race, with team-mate Mari Boya swerving to avoid him and suffering terminal damage to his rear-left side.
Racing resumed on lap six, with Franco Colapinto also making it past Barnard, this time up the inside of the Fairmont hairpin.
Tommy Smith was involved in a shunt heading into the tunnel in his Van Amersfoort Racing car, with Oliver Goethe bumping him into the wall and receiving a 10-second penalty.
Smith was also involved in a tussle between Piotr Wisnicki and Ido Cohen at the hairpin as the trio battled further down the field, the Israeli driver nudging the Pole as they headed down the hill.
By lap 15, the gap from Marti to Fornaroli was two seconds, and it continued to grow, up to three seconds just two laps later as the Spaniard pulled away and Fornaroli fell into Saucy's clutches.
Further back, Sebastian Montoya (Hitech) looked set to close on Trident driver Gabriel Bortoleto, but the Brazilian was able to hold him behind in the fight for sixth.
Saucy had a small lock-up into the hairpin on lap 21 as he attempted to close further on Fornaroli, but never made it close enough.
Colapinto finished fourth with Barnard and Bortoleto in fifth and sixth. Montoya, Browning, Collet and Prema driver Paul Aron rounded off the top 10.
| Cla | Driver | Team | Gap |
|---|---|---|---|
| 1 | Pepe Martí |  Campos Racing |
|
| 2 | Leonardo Fornaroli |  Trident |
8.100 |
| 3 |
 Gregoire Saucy |
 ART Grand Prix |
8.800 |
| 4 |
 Franco Colapinto |
 MP Motorsport |
9.300 |
| 5 | Taylor Barnard | Jenzer Motorsport |
11.500 |
| 6 | Gabriel Bortoleto | Trident |
13.600 |
| 7 |
 Sebastian Montoya |
Hitech Pulse-Eight | 13.900 |
| 8 | Luke Browning | Hitech Pulse-Eight | 14.500 |
| 9 |
 Caio Collet |
Van Amersfoort Racing |
15.900 |
| 10 | Paul Aron | Prema Powerteam |
17.900 |
| 11 | Gabriele Minì | Hitech Pulse-Eight | 18.400 |
| 12 | Dino Beganovic | Prema Powerteam |
19.900 |
| 13 |
 Zak O'Sullivan |
Prema Powerteam |
20.900 |
| 14 | Nikola Tsolov | ART Grand Prix |
22.200 |
| 15 | Nikita Bedrin | Jenzer Motorsport |
23.200 |
| 16 |
 Rafael Villagomez |
Van Amersfoort Racing |
24.000 |
| 17 | Oliver Goethe | Trident |
35.800 |
| 18 | Piotr Wiśnicki | PHM Racing by Charouz | 38.400 |
| 19 |
 Ido Cohen |
Rodin Carlin | 40.300 |
| 20 | Christian Mansell | Campos Racing |
40.800 |
| 21 | Tommy Smith | Van Amersfoort Racing |
42.300 |
| 22 | Oliver Gray | Rodin Carlin | 43.700 |
| 23 |
 Sophia Flörsch |
PHM Racing by Charouz | 45.100 |
| 24 |
 Kaylen Frederick |
ART Grand Prix |
47.300 |
| 25 | Hugh Barter | Campos Racing |
50.000 |
| 26 | Alex Garcia | Jenzer Motorsport |
1'01.300 |
| 27 | Hunter Yeany | Rodin Carlin | |
| Roberto Faria | PHM Racing by Charouz | ||
| Mari Boya | MP Motorsport |
||
|
Jonny Edgar |
MP Motorsport |
||
| View full results | |||
