GT convert Rossi contested the two Road to Le Mans support races that form part of the Le Mans Cup championship in a Team WRT BMW M4 GT3 that he shared with Jerome Policand - who admitted he originally thought the chance to partner the Italian was a joke.
Rossi briefly led the opening race on Thursday before getting baulked at the first Mulsanne chicane by two LMP3 cars that tangled directly in front of him.
The pair eventually finished 13th in class after slipping further down the field, before Rossi then charged up the order in race two.
He enjoyed a close scrap with Anders Fjordbach's Porsche in the later stages for second, diving ahead with a lap to go, and then was promoted to the win when the AF Corse Ferrari of Kei Cozzolino and Hiroshi Koizumi was penalised for "erratic driving".
“It was the first victory together with BMW, and it was also a great experience to come here to prepare for next year’s 24-hour race," said Rossi, who intends to be on the grid when the new GT3 class is introduced in 2024 to replace GTE.
"We had a strong pace since yesterday morning and in qualifying, but we were a bit unlucky in the first race, so we wanted to go for the podium in the second race.
"Jerome had a very good first stint and, at the end of the day, we were able to win.
"I had a good pace and was able to do some good overtakes. We are very, very happy.”
Andreas Roos, head of BMW M Motorsport, added: "We hope to be able to build on these successes next year. This was a first step."
In each of the crash-strewn races, the overall winner on the road was handed a penalty with the Nielsen Racing Duqueine LMP3 of Matt Bell and John Melsom being promoted to the opening victory and the Cool Racing Ligier of David Droux and Luis Sanjuan taking the honours in the second bout.
