Yuika Sugasawa's clinical near-post flick in the 89th minute gave hosts Japan the title on goals scored after a 1-1 opening draw with the Americans, who beat Brazil 3-0.
"The players kept pushing and never stopped battling," Japan coach Norio Sasaki told reporters. "That's what this team is all about.
"Winning the title is a boost and with 111 days to the London Olympics, hopefully there's a lucky omen in that for us."
But Japan regained the lead through Yuko Nagasato's header 13 minutes into the second half, before captain Aya Miyama, pictured, fired a third moments later as the home side took control.
Named after a frilly pink flower, Japan's 'Nadeshiko' stunned the mighty U.S. on penalties in the World Cup final in Frankfurt last July to become the first Asian side to win a world title.
Their astonishing World Cup victory gave a traumatised nation a psychological boost after last year's deadly tsunami, which triggered a nuclear crisis north of Tokyo.
"It was a team effort tonight," said Miyama. "We're not quite at 100 percent yet but we're positive about peaking for London. We want to prove we're the world's best again."
