Reeves was born in Beirut, Lebanon, the son of Patricia Taylor, a costume designer/performer, and Samuel Nowlin Reeves, Jr., a geologist. Reeves' mother is English, and his father is American of Hawaiian-Chinese, Irish and Portuguese descent. Reeves's mother was working in Beirut when she met his father. Reeves' father worked as an unskilled laborer and earned his GED while imprisoned in Hawaii for selling cocaine at the Hilo airport. He abandoned his wife and family when Reeves was 13, and Reeves does not currently have any relationship with him. Reeves is named after his uncle, Henry Keanu Reeves. When Reeves first arrived in Hollywood, his agent thought his first name was too exotic, so during the early days of his film career he was sometimes credited as K.C. Reeves.

Reeves has one full sister named Kim (born 1966 in Australia) who was diagnosed with leukemia in the early 1990s. Additionally, through his mother he has a half-sister named Karina Miller (born 1976 in Toronto