The present study investigated the antioxidant activity and components of hydrothermal extracts from watermelons. Hydrothermal treatment of watermelons significantly increased the phenolic content and antioxidant activity of the flesh, green rind, and white rind parts of the fruit. Several new phenolic compounds such as catechol and its derivatives were identified in the hydrothermal extracts. Therefore, hydrothermal treatment may release phenolic compounds from watermelons, thereby increasing its antioxidant activity. This study shows that hydrothermal treatment could be used as a tool to increase the antioxidant activity and levels of nutraceutical compounds in watermelon extracts.