The accident occurred when Bett's vehicle hit a bump on the road between Eldoret and Kapsabet and rolled over.
"He died on the spot," said Nandi county police commander Patrick Wambani.
"Our condolences to his family and the entire athletics fraternity," Athletics Kenya said on social media.
The IAAF, athletics' world governing body, added: "The IAAF is deeply saddened and shocked to hear that Kenya’s 2015 world 400m hurdles champion Nicholas Bett, 28, died in a car accident. RIP."
As a youngster, Bett's preferred sport was volleyball. After switching to athletics, he initially showed promise in the 110m hurdles but gravitated towards the longer event. He coached himself for several years into a 49-second performer.
In 2014 the Kenyan coach Vincent Mumo introduced the then 24-year-old Bett to Jukka Harkonen, who became his agent and organised a link-up with the South African coach Hennie Kotze.
Bett’s training stints in Finland and South Africa led to significant technical changes in the way he approached the event.
Success followed in Beijing where he surprised the field by taking gold.
Before his triumph, Kenya had won 45 gold medals at the 14 world championships since 1983. However, none of them had been in an event shorter than 800m.
Bett finished in a Kenya national record of 47.79 seconds. Denis Kudryavtsev from Russia was second and Jeffery Gibson from the Bahamas claimed the bronze.
But since that high in China, Bett failed to reproduce comparable form. He was disqualified in his heat at the 2016 Rio Olympics after hitting a hurdle and injury prevented him from defending his world title in London in 2017.
He was unable to take part in last week's African championships in Asaba, Nigeria, due to injury.
"It is very sad," said Barnaba Korir, an official with Athletics Kenya. "He had gone to Nairobi Hospital to see a doctor because of the injury he picked up in Asaba.
"We are saddened beyond words."