The former Formula 1 driver, now 38, posted the news on Instagram.
The couple have been together since 2016, three months after Button's first marriage ended.
Button married former partner Jessica Michibata in 2014, but he split from the Japanese model just 12 months after their wedding in Hawaii.
Previously, he had been engaged to actress and singer Louise Griffiths, but their five-year relationship ended before taking the plunge.
Ward, a California native, appeared in Playboy for the first time in 2015 as Miss January.
Button now competes in Japan's Super GT Series for Honda, the company he raced for between 2006 and 2008 in Formula 1.
He clinched the World Championship title in 2009 after the Japanese marque became Brawn, reportedly taking a 50 per cent pay cut to remain with them after Honda quit the sport due to the 2008 financial crisis.
Eight seasons with McLaren followed before Button quit Formula 1 for good last year.
He will compete for SMP Racing in the 2018-19 FIA World Endurance Championship, as well as the 2018 24 Hours of Le Mans, which starts this Saturday.
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