The race was run over 70 laps of the 4.262 km Zolder circuit, a total race distance of 298.340 km. Weather on race day was dry. The Belgian Grand Prix was a pivotal moment in the 1978 season: Andretti had been consistently quick across the first five rounds and arrived as championship leader, and the race represented the public debut of the fully developed Lotus 79 ground-effect car. Andretti drove the new Lotus 79, while teammate Peterson used the older Lotus 78.
The event also introduced a pre-qualifying session for marginal or less-established entries, an early version of what became a regular feature of late-1970s and 1980s grand prix weekends.
Seven cars contested the pre-qualifying session. Rolf Stommelen set the fastest time at 1:23.76 in an Arrows-Ford. Keke Rosberg (Theodore-Ford), René Arnoux (Martini-Ford), Bruno Giacomelli (McLaren-Ford), and Brett Lunger (McLaren-Ford) all advanced to the main qualifying session. Héctor Rebaque and Arturo Merzario failed to advance.
Andretti claimed pole position with a lap of 1:20.90 in the new Lotus 79, immediately underlining the car's pace advantage. Carlos Reutemann qualified second for Ferrari at 1:21.69, Niki Lauda third for Brabham-Alfa Romeo at 1:21.70, and Gilles Villeneuve fourth for Ferrari at 1:21.77. Jody Scheckter was fifth for Wolf-Ford, with James Hunt sixth for McLaren and Peterson seventh for Lotus at 1:22.62. Alberto Colombo entered for the Formula One World Championship debut.
At the start, Reutemann made a poor getaway from second on the grid, causing a chain reaction at the first corner. Niki Lauda was struck by Scheckter's Wolf and both were retired before completing a lap. James Hunt in the McLaren and Emerson Fittipaldi were also eliminated at or near the first corner; all four were classified as lap-zero retirements.
Andretti took the lead immediately and was never seriously threatened. Villeneuve emerged second from the opening-lap chaos, with Peterson third. Neither Ferrari nor the older Lotus could live with Andretti's pace. The first 40 laps ran without major incident until Villeneuve suffered a puncture around lap 40 and pitted, dropping him back to fifth. A few laps later Peterson also pitted for fresh tyres, leaving Reutemann second and Jacques Laffite's Ligier-Matra third.
On new rubber Peterson drove quickly through the field, passing both Laffite and Reutemann to move into second. On the final lap Laffite attempted to take third from Reutemann, but the two made contact and Laffite was eliminated. Reutemann survived to take third. Villeneuve recovered to finish fourth despite his mid-race pit stop. Laffite was classified fifth under accident, and Didier Pironi took sixth for Tyrrell-Ford from 23rd on the grid.
Andretti won in 1 hour 39 minutes 52.02 seconds. Peterson finished second 9.90 seconds behind, completing a Lotus 1–2. Reutemann was a further 14.44 seconds back in third. Peterson set the fastest race lap at 1:23.13 on lap 66.
The victory extended Andretti's Drivers' Championship lead to 27 points, with Patrick Depailler second on 23 and Reutemann third on 22. Peterson moved to fourth on 20 points. In the Constructors' Championship, Lotus led comfortably on 36 points ahead of Tyrrell on 25 and Ferrari on 22. The race reinforced the extent to which Lotus had pulled clear of the rest of the field through the aerodynamic advantage of their ground-effect chassis, with the Lotus 79 in particular proving a significant step forward from the Lotus 78.