Add the watermelon, sugar, and water to a pot over a medium/low heat and stir.
Bring to a boil, then reduce the heat and simmer for 10 minutes.
Remove from the heat and cool slightly.
Place a sieve over a bowl and carefully pour in the syrup to strain out the watermelon (tip: you can save it and serve the sweet watermelon cubes on ice cream, yogurt or pancakes!).
Allow the syrup to cool and pour into a sanitized bottle or jar.