According to Richmond Heights police, the shooting happened just after 12:40 a.m. at an apartment complex in the 7200 block of West Park Avenue, near Bellevue Avenue.
Officers found the victim, a 31-year-old man, dead in an apartment with an apparent gunshot wound. Police also found the victim's 29-year-old girlfriend, who allegedly admitted to shooting the victim during a domestic violence incident.
The woman was taken into custody.
Police have turned the case over to the St. Louis County Prosecuting Attorney's Office for any potential charges.
The identity of the victim is being withheld pending notification of family members.