Cheptegei, 33, is receiving treatment for burns to 75% of her body and is "fully sedated because of the extent of the burns," according to her doctors.
The marathon runner was attacked at her home Sunday in western Kenya after returning home from church with her two children.
"The couple were heard quarreling outside their house in the small town of Endebess. During the altercation, the boyfriend was seen pouring a liquid on the woman before burning her," local police chief Jeremiah Ole Kosiom told reporters.
"The man was also caught in the same fire and sustained serious burns," Kosiom added.
A local administrator in Trans Nzoia county, which is close to Kenya's athletic training centers, said the couple had been fighting over a piece of land Cheptegei purchased to build her house.
Police said Cheptegei's former boyfriend, Dickson Ndiema Marangach, poured gasoline on her and also was burned in the fire. Both are receiving treatment at the Moi Teaching and Referral Hospital in Eldoret city.
"We regret to announce that our athlete Rebecca Cheptegei, who competed at the Olympics, has suffered severe injuries and is hospitalized at Moi Teaching and Referral Hospital in Eldoret. This follows an incident involving her Kenyan boyfriend pouring petrol and setting fire on her," the Uganda Athletics Federation wrote in a post on X.
Cheptegei finished 44th in the marathon at last month's Olympics. She also won gold at the World Mountain and Trail Running Championships in Chiang Mai Thailand two years ago. Cheptegei is not the only athlete in Kenya to be targeted by a partner.
Kenyan-born Bahraini runner Damaris Mutua was found strangled in 2022 in the Rift Valley town of Iten. Months earlier, record-breaking long-distance runner Agnes Tirop was found stabbed to death in the same town.
Ugandan Olympic runner and steeplechaser Benjamin Kiplagat was stabbed to death by assailants in Eldoret last year. By Sheri Walsh, UPI