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."
Brazil captain Daiane headed an own goal to give Japan an early advantage in the 16th minute in windy Kobe.
The South Americans equalised on the stroke of halftime when midfielder Francielle curled in a superb free kick.
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.
Sugasawa darted in front of her marker to meet a superb cross from right-back Yukari Kinga to add the late fourth and sting the world number one Americans once again.
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."
