Despite being passed by Jack Doohan for the lead off the line, the DAMS driver reclaimed the front spot at the North chicane on lap 1 before building a huge lead over his rivals.
A quick pitstop helped his charge to victory, and the Japanese driver put no foot wrong in the second stage, holding off Theo Pourchaire to take his first series win.
Pourchaire took second at his home race for ART, moving past Doohan on lap 15 and keeping the Australian behind him, despite a close call with the Virtuosi driver spinning on lap 20.
Teammate Frederik Vesti held off a charging Felipe Drugovich for third.
Lining up fourth, Doohan claimed the lead off the line up the inside, running ahead of Iwasa and Logan Sargeant.
But the Japanese driver took charge of the race on the first lap before an early safety car courtesy of contact between Marcus Armstrong and Dennis Hauger at Turn 12, ending the Hitech driver's race and prompting a safety car.
Racing resumed on Lap 6, with Liam Lawson claiming seventh from Drugovich down the inside of Turn 1, while out front Iwasa had built up over a second gap to Doohan.
MP Motorsport's Clement Novalak was first to dive into the pits on Lap 7, taking the soft tyres, before several other drivers followed over the next few laps.
Pourchaire pitted for the hard tyres on Lap 10 and emerged in 12th on track, leading those who had pitted, before Doohan and Drugovich followed suit three laps later.
The Frenchman made it past Doohan the following lap, taking advantage of the latter's cold tyres to move into net second.
Heartbreak came for Sargeant during his pitstop, with an engine issue ending his race having been running in the podium places and handing Pourchaire second in the drivers' standings.
But the Australian fought back, mounting a challenge at the North chicane with 10 laps remaining, before spinning across the kerb and losing his position to Vesti.
By that point, Iwasa had built up a lead of 4.7s to Pourchaire, who escaped contact with Doohan.
Jehan Daruvala took two positions in as many laps in the late stages to move up to seventh, passing Richard Verschoor and Roy Nissany.
Doohan was facing pressure from charging championship leader Drugovich by the final four laps, the Brazilian eventually making it past at Turn 8 on Lap 27.
More bad luck came for Verschoor, running in eighth, on the final lap, as his Trident car suffered a terminal technical issue and he came to a stop on the main straight.
Drugovich settled for fourth, with Doohan taking fifth having led at the start. Lawson took sixth as Carlin's only finisher, with Daruvala and Nissany rounding off the points.
Drugovich continues to lead the standings with five rounds remaining, while Pourchaire reclaims second after Sargeant's DNF.
| Cla | Driver | Team | Gap |
|---|---|---|---|
| 1 |
 Ayumu Iwasa |  DAMS | |
| 2 |
Theo Pourchaire | ART Grand Prix | 8.600 |
| 3 |
 Frederik Vesti | ART Grand Prix | 9.800 |
| 4 |
 Felipe Drugovich |  MP Motorsport | 10.200 |
| 5 |
 Jack Doohan |  UNI-Virtuosi | 16.000 |
| 6 |
 Liam Lawson | Carlin | 19.600 |
| 7 |
 Jehan Daruvala |  Prema Powerteam | 27.500 |
| 8 |
Clement Novalak | MP Motorsport | 32.800 |
| 9 |
 Roy Nissany | DAMS | 36.500 |
| 10 |
Enzo Fittipaldi |  Charouz Racing System | 37.600 |
| 11 |
Calan Williams | Trident | 40.200 |
| 12 |
 Jüri Vips | HitechGP | 45.500 |
| 13 |
Olli Caldwell |  Campos Racing | 46.400 |
| 14 |
 David Beckmann | Van Amersfoort Racing | 48.500 |
| 15 |
 Amaury Cordeel | Van Amersfoort Racing | 49.000 |
| 16 |
 Dennis Hauger | Prema Powerteam | 1'25.100 |
| 17 |
Richard Verschoor | Trident | |
 Cem Bolukbasi | Charouz Racing System | ||
 Logan Sargeant | Carlin | ||
|
Roberto Merhi | Campos Racing | ||
|
Marcus Armstrong | HitechGP | ||
|
Marino Sato | UNI-Virtuosi | ||
| View full results | |||
