She received the award for her role in "By the Stream," also known as "Suyoocheon" in Korean.
Directed by Hong Sang-soo, the film deals with the story of a university lecturer asking her unemployed uncle to direct a skit.
In her acceptance speech last Saturday, Kim expressed her gratitude to her lover Hong, saying that Hong made "such a beautiful film" and that she loved his movies. The actress has been involved in a May-to-September romance with Hong since 2016.
The festival, held annually in the resort city of Locarno in southern Switzerland in August, is known for showcasing innovative and experimental films around the world. Since last year, the festival has presented a single award for best performance without distinguishing between male and female categories.
