Land Rover BAR eked out a third-place finish in the event's final race to secure the title at the Fukuoka regatta after Artemis Racing of Sweden won the day's first two races.
The British crew extended their lead in the season's World Seriesleaderboard with 512 points, just ahead of two-time defending America'sCup champions Oracle Team USA with 493.
"It's a real bonus for our team... to show we can perform at the highest level," Ainslie told AFP.
"Now, we focus on our race boat, the design and development of the race boat and performing well next summer in Bermuda."
Five challengers are vying for the chance to take on Oracle Team USA in the America's Cup final in Bermuda next year.
Ainslie's team have their headquarters in Portsmouth and are at the centre of a multi-million pound enterprise, funded by some of the country’s wealthiest businessmen aiming to bring the America’s Cup to Britain for the first time.
Fukuoka is the final stage of a protracted qualifying process that counts towards the 2017 America's Cup Challenger Series, the winner of which will take on Oracle in the 2017 America's Cup in Bermuda.
The America's Cup dates back to 1851 when the yacht America won a race around the Isle of Wight off the south coast of Britain.
Only teams from four countries -- the United States, Switzerland, New Zealand and Australia -- have managed to win the America's Cup since then.
