In a curtain-raiser for the Formula 1 Grand Prix later in the day, the 19-year-old overtook Australian Christian Mansell on the final corner of the 10-race season to grab third place and snatch the title.
That manoeuvre gave Fornaroli five extra points. He edged Gabriele Mini, who was second in the race and the season standings, by two points, to become the first Italian to take the F3 title.
Fornaroli's French teammate at Trident, Sami Meguetounif, won the race.
Fornaroli was second entering the penultimate lap but both Mini and Mansell overtook.
"Not my race actually I have to say because I did plenty of mistakes," Fornaroli said.
"We managed to recover at the end in the last corner to get third place and the championship as well. It's been a great season, even without a win, we managed to win the title."
