The lion attacked the 45-year-old Ukrainian late Sunday after he used a rope to climb down into an enclosure with four lions, said Kiev police spokesman Volodymyr Polishchuk.
He said the man, who was not identified, was acting aggressively and the lion seized him by the throat. The man, an ethnic Azerbaijani, died at the scene.
Ukrainian TV channel NTN broadcast interviews with witnesses who said the man told them that he wanted to test God, believing that God would not allow the lions to hurt him.
Zoo officials could not immediately be reached for comment.