From third on the grid Rossi jumped ahead of Nathanael Berthon into La Source and quickly set about catching poleman King.
Rossi looked to have the lead down the Kemmel straight but King defended well, only to run wide coming out of Les Combes and gift the place to Rossi.
The American managed the gap for the majority of the race, although King did managed to bring it down to as low as 0.6 seconds towards the end.
Vandoorne: I had to dominate GP2
Rossi was never under any serious pressure and crossed the line 1.5s clear of his team-mate to take his first GP2 win since Abu Dhabi 2013.
King was comfortably second, despite a few laps of intense pressure from McLaren protege Stoffel Vandoorne's ART car.
Vandoorne however was soon under pressure himself from Mitch Evans after locking up at the Bus Stop chicane on the restart from a virtual safety car caused by Arthur Pic beaching his Campos Racing car in the gravel at Stavelot.
Evans, who had a slow getaway on the restart, passed Vandoorne after a handful of attempts down the Kemmel straight in the closing stages to secure his first podium of the season.
His Russian Time team-mate Artem Markelov capped off a weekend that also featured his first podium finish with a strong race in fifth, passing Sergey Sirotkin in the closing stages.
Lazarus driver Berthon started on the front row but couldn't hold on to a podium place. He fell slowly fell down the order to seventh despite a great start.
Williams Formula 1 development driver Alex Lynn had an eventful race with Sergio Canamasas to finish eighth.
The pair battled hard for the majority of the distance before Lynn eventually found a way past.
Rio Haryanto rounded out the top 10 ahead of Julian Leal, who fell dramatically down the order after starting fifth on the grid as he struggled with a lack of DRS.
RESULTS - 18 LAPS:
Vandoorne seals GP2 title in Russia
Rossi wins after Lynn crashes out