Hanescu, who gained a wild card entry into the event, enjoyed the support of the home crowd in his first ATP final. He also was on the brink of victory after taking the first set Sunday.
But Simon roared back to post a 4-6, 6-3, 6-2 win.
It was the sixth-seeded Simon's ATP crown of the year; he previously beat Marcos Baghdatis of Cyprus in the title match at Marseille in February.
Hanescu was trying to become the first Romanian to win an ATP event since Andrei Pavel won the Montreal Masters in August 2001.