DAMS racer Sainz Jr kept his composure during a frantic opening to the race to pull away from his rivals with ease, eventually winning by 18 seconds, with Jazeman Jaafar pouncing on a last-lap mistake by Matias Laine to claim third.
An extra formation lap was called due to a failure of the starting lights but the problem was not resolved in time and proceedings were halted for a few minutes.
Carlos Sainz Jr maintained his advantage at the start but behind him the field concertina effect at Les Combes resulted in chaos. Richie Stanaway made contact with Roberto Merhi, the resulting collision tipping Sergey Sirotkin into the air before he hit the barriers, while Zoel Amberg was clipped into a spin. All four drivers were forced to retire and the safety car was deployed to clear up the mess. Fortec's Oliver Rowland, who arrived in Belgium as Sainz Jr's nearest rival, was also caught up but continued in last place.
Sainz Jr stormed away at the restart but in the midfield Pietro Fantin collided with Meindert van Buuren at Les Combes, leaving the former in the barrier and the latter in a spin. Rowland made up a few positions but his car slowed with around 10 minutes of the race remaining and he was forced to retire.
Sainz's winning margin was emphatic, while Gasly led a train of cars over the line which included Jaafar, Laine and Norman Nato, who slipped back following a bold pass on Jaafar early in the race.
Luca Ghiotto claimed sixth place with a fabulous late pass on Marco Sorensen into Eau Rouge, while Will Stevens was eighth. Marlon Stockinger and Beitske Visser rounded out the top 10.