The game went to extra-time following a tense encounter that saw neither team gain more than a one-goal advantage.
Martina Miceli and Melania Grego were the stars for Italy grabbing three goals apiece, with Grego scoring the winner with just over two minutes left.
Meanwhile, centre-fielder Ellen Estes scored the decider to help the United States to a 6-5 victory and the bronze medal at the expense of Australia.