There was another bumper entry for the Belgian GP with a total of 32 cars fighting for the 26 grid positions. Team Surtees had dropped Hans Binder and hired Larry Perkins instead, while the other new entries were Bernard de Dryver in Brian Henton's March and Mexico's Héctor Rebaque in a Marlboro-sponsored third Hesketh. James Hunt was back in an M26 and BS Fabrications had acquired a McLaren M23 for Brett Lunger while Fittipaldi had the new FD5 on hand for the first time. having failed to qualify a car at Monaco, March had the new 771 ready for Ian Scheckter to drive.

Qualifying took place in overcast and wet conditions and resulted in a dominant pole position for Mario Andretti in the Lotus 78, the American 1.5 secs faster than John Watson's Brabham-Alfa Romeo. The second Lotus driver Gunnar Nilsson was third with Jody Scheckter fourth in his Wolf (which had won at Monaco a fortnight earlier). Behind them came Patrick Depailler (Tyrrell), Jochen Mass (McLaren), Carlos Reutemann (Ferrari) and Ronnie Peterson (Tyrrell) while the top 10 was completed by James Hunt in his difficult M26 and Jacques Laffite in the Ligier-Matra. Niki Lauda was 11th in his Ferrari.

It was raining on Sunday afternoon as the start of the race approached but it was not certain it would continue and while most of the field was on wet tyres, Hunt chose slicks. Watson made the best start and took the lead from Andretti. The field got through the first corner without any major adventures although both Ian Scheckter and Harald Ertl spun but at the chicane behind the paddock Andretti made a mistake and ran into the the back of Watson. Both spun off. Nilsson had to swerve to avoid the incident and this allowed Scheckter to take the lead. Nilsson was second with Mass third and Reutemann fourth. Behind them came Depailler, Laffite, Peterson and Lauda. Hunt's tyre gamble was a disaster and he was dropping away quickly.

On the ninth lap Reutemann overtook Mass for third but a few laps later he spun into retirement and Mass moved back to third. By then the track was drying and there was a series of pit stops. Lauda stopped early and when the stops were over he was ahead with Mass second and Alan Jones third in his Shadow. Nilsson had lost a lot of ground because of sticking wheelnut and was eighth behind Brambilla, Scheckter, Laffite and Peterson.

He overtook Peterson, Laffite and Brambilla, while Scheckter pitted when the rains came again. Mass spun out on lap 40 and so Nilsson was back in second place behind Lauda. he began to close as the track dried again (sending Scheckter back to the pits) and on the 50th lap Nilsson took the lead and drove away to win his first Grand Prix victory. Lauda was second with Peterson third after a battle with Brambilla. Jones finished fifth and Stuck sixth.