By Agence France-Presse
It is rare for police to shoot suspects in Hong Kong, where it is illegal for the public to carry firearms.
Officers intercepted a "suspicious" man in Sham Shui Po station but he then pulled a six-inch (15-centimetre) knife from his backpack, divisional commander Chow Ngai-kong told reporters.
After giving him a warning, a policewoman shot him and he was sent to hospital with a stomach injury.
The 55-year-old knifeman, who has been arrested, is in a critical condition.
Asked by reporters why police had fired at the man during rush hour in a crowded station, Chow said their lives had been under threat.
"The male was using a life-threatening sharp-edged weapon to try to attack officers at the scene," Chow said.
"She has considered whether she has the opportunity to hit the target before she actually opened fire," he added.
The man was a Hong Kong identity card holder, Chow said, giving no further detail on the suspect.