The DAMS driver lined up second behind fellow Red Bull junior Isack Hadjar, holding his place off the line.
But on the safety car restart on lap six, Hitech driver Hadjar suddenly slowed, with a mechanical issue forcing him to drop down the order before retiring.
Iwasa inherited the lead and began building a substantial gap, up to 2.2s by lap 10, before racing was again neutralised on lap 22.
He led the restart before again increasing the gap to second-placed Jehan Daruvala (MP Motorsport), taking the win by 6.6s.
Jak Crawford finished third for Hitech, his third podium of his rookie F2 season.
The start went smoothly, with reverse-grid polesitter Hadjar leading from Iwasa and Daruvala off the line, while Victor Martins passed ART team-mate Theo Pourchaire at the hairpin for eighth.
But chaos erupted once the back of the field reached the Nouvelle Chicane, with a four-car incident prompting the first safety car.
Trident driver Clement Novalak made contact with Kush Maini (Campos) as the latter battled with Amaury Cordeel (Invicta Virtuosi).
DAMS' Arthur Leclerc, Dennis Hauger (MP), PHM Racing driver Roy Nissany and Ralph Boschung (Campos) were all caught up in the incident, with Leclerc, Hauger and Novalak all forced to pit while Boschung and Nissany's races were ended.
Novalak received a 10-second penalty for the incident, before receiving another five-second penalty for speeding in the pitlane.
Racing resumed on lap six, with Hadjar's issue arising almost immediately upon the restart, forcing him out of the lead and forcing him back to the pits to retire.
Iwasa was 2.2s clear of Daruvala by lap 10, who was in turn 5.8s ahead of third-placed Crawford, though the MP driver began to close the gap, cutting it to 1.4s by lap 16.
Enzo Fittipaldi (Rodin Carlin) passed Cordeel for 10th on lap 22 but moments later the Belgian ended up in the wall, having made contact with Juan Manuel Correa (VAR) at Turn 5, forcing another safety car.
The safety car ended on lap 25, with Prema rookie Oliver Bearman retiring with an unknown issue.
Iwasa immediately rebuilt the gap to Daruvala, and was five seconds clear by lap 27 before crossing the line 6.6s clear.
Richard Verschoor finished fourth for Van Amersfoort Racing, with Rodin Carlin's Zane Maloney in fifth and Jack Doohan (Invicta Virtuosi) in sixth.
The ART pair of Martins and Pourchaire were seventh and eighth, with Frederik Vesti in ninth for Prema and Fittipaldi rounding off the top 10.
| Cla | Driver | Team | Gap |
|---|---|---|---|
| 1 |
 Ayumu Iwasa |
 DAMS |
|
| 2 |
 Jehan Daruvala |
 MP Motorsport |
6.678 |
| 3 |
 Jak Crawford |
Hitech Pulse-Eight | 8.335 |
| 4 |
Richard Verschoor |
Van Amersfoort Racing |
8.820 |
| 5 |
 Zane Maloney |
Rodin Carlin | 10.978 |
| 6 |
 Jack Doohan |
Invicta Virtuosi Racing | 14.635 |
| 7 |
Victor Martins |
ART Grand Prix |
15.389 |
| 8 |
Theo Pourchaire |
ART Grand Prix |
17.369 |
| 9 |
 Frederik Vesti |
 Prema Powerteam |
18.411 |
| 10 |
 Enzo Fittipaldi |
Rodin Carlin | 18.801 |
| 11 |
Juan Manuel Correa |
Van Amersfoort Racing |
19.303 |
| 12 |
Brad Benavides |
PHM Racing by Charouz | 20.541 |
| 13 |
 Roman Staněk |
Trident |
21.352 |
| 14 |
Kush Maini |
 Campos Racing |
22.395 |
| 15 |
 Arthur Leclerc |
DAMS |
22.962 |
| 16 |
 Dennis Hauger |
MP Motorsport |
25.049 |
| 17 |
Clement Novalak |
Trident |
38.434 |
 Oliver Bearman |
Prema Powerteam |
||
 Amaury Cordeel |
Invicta Virtuosi Racing | ||
|
Isack Hadjar |
Hitech Pulse-Eight | ||
 Roy Nissany |
PHM Racing by Charouz | ||
 Ralph Boschung |
Campos Racing |
||
| View full results | |||
