Koum was born in Kyiv, Ukraine, then in the Soviet Union. He raised and grew up in Fastiv, outside Kyiv, and moved with his mother and grandmother to Mountain View, California. Koum and his mother remained in touch with his father until his death. Koum's mother worked as a babysitter, while he himself began as a cleaner at a grocery store. His mother died in 2000 after a long battle with cancer.