Yes, watermelon is a berry. This might surprise you because we usually think of berries as being small, but in the botanical world, the definition of a berry is a fruit that develops from a single ovary and contains multiple seeds. Watermelons fit this definition perfectly, as they have a fleshy, juicy interior with multiple seeds and develop from a single ovary.
While we may consider strawberries and blueberries to be berries, botanically they are not. Strawberries are technically "aggregate fruits," which are formed from multiple ovaries, and blueberries are classified as "drupes," which are fruits with a single seed surrounded by a hard pit.
So, the next time you're enjoying a slice of watermelon, remember that you're eating a giant berry!