The 28-year-old world record holder pulled clear from the field early on and led by around 40 metres at the bell before coasting home in 8min 57.84sec to win gold.
Defending champion Emma Coburn of the United States took silver with a time of 9:02.35 ahead of Germany's Gesa Krause, who claimed bronze in 9:03.30.
It was a sweet victory for Chepkoech, the dominant force in her event this season.
The win also erased the memory of Chepkoech's 2017 World Championships, when she missed a water jump early in the race and was forced to backtrack to clear it.
That gaffe saw her finish out of the medals in fourth spot as Coburn took gold.
But there was to be no repeat at the Khalifa Stadium in Monday's final as the Kenyan cranked up the pace to pull away to leave her rivals trailing.