The second-seeded Italian, 28, needed a second-set tie-break to stay alive before comfortably taking the decider to triumph in two hours and 26 minutes.
It was Seppi’s third career title and second this season as he matures into a genuine contender in his late 20s – he is set to achieve a career-high ranking of 22 following this victory.
"Thomaz started well and was serving very good, while I experienced problems on return," Seppi said. "But I tried hard to stay close and put pressure on him.
"He played two bad games when he was serving for the match in the second set and it added me some confidence and helped me to win the tie-break and especially in the third set."
Brazilian fourth seed Bellucci, 24, had taken the first set with a fourth-game break and, after trading breaks with his more experienced opponent, failed to serve for the match as the Italian forced a tie-break.
It was Bellucci’s first career hard court final, having won twice in Gstaad, once in Santiago and lost in the Brazil Open final.