BMW factory driver Rossi finished runner-up in the second of the two GTWCE Sprint Cup races at the British venue aboard the WRT BMW M4 GT3 he shared with Maxime Martin.
Rossi, who is in his second full season of car racing after calling time on his two-wheel career, finished just under four seconds behind the winning Attempto-run Audi R8 LMS GT3 shared by Ricardo Feller and Mattia Drudi at the end of the one-hour race.
Martin started third in their BMW and pitted out of that position.
Rossi emerged from the tyre stops that followed a brief safety car period in second behind Drudi, who had moved up from sixth before the stops.
He initially had reigning champion and team-mate Charles Weerts on his tail in one of the WRT sister cars before establishing some breathing space.
Weerts, who shared his BMW with fellow three-time Sprint Cup champion Dries Vanthoor, came back at Rossi in the closing stages, taking the chequered flag just 0.8s behind.
Martin and Rossi benefitted from problems encountered by the two Audis that ran 1-2 in the early stages.
#46 Team WRT, BMW M4 GT3: Valentino Rossi, Maxime Martin
Photo by: SRO
The Sainteloc entry in which Christopher Mies led the first 23 of the 41 laps lost vital seconds during the pitstops on the handover to Gregoire Demoustier, who briefly took to the gravel at Sheene Curve on his out-lap.
Christopher Haase had shadowed them in the early stages aboard his Cometoyou Racing, which also lost time in the pits.
Lucas Legeret ended up fifth in the Cometoyou car, holding off a late challenge from Raffaele Marciello in the Auto Sport Promotion Mercedes-AMG GT3 in which he had won the opening race at Brands together with Timur Boguslavskiy.
Boguslavskiy had qualified only 19th for race two and made little progress in the opening phase of the race, before a super-fast pitstop by ASP set Marciello up for his late charge.
Marciello had dominated the first half of race one, building up a lead of eight seconds before pitting.
The advantage held by Boguslavskiy stretched out to 15s after the stops courtesy of a problem for the Emil Frey Ferrari 296 GT3 in which Albert Costa had led the chase of Marciello.
Feller was able to close down the gap in the Attempto Audi, but just failed to catch the Boguslavskiy and ended up six tenths down at the finish.