Cube the watermelon into 1/2-inch pieces.

Place watermelon cubes in a blender.

Blend until smooth, creating a watermelon puree.

In a small pitcher, combine fresh mint leaves, sugar, and fresh lime juice.

Using the handle of a wooden spoon, gently muddle the ingredients.

Release the oils from the mint, being careful not to over-muddle and tear the leaves.

Pour light rum into the pitcher.

Add the watermelon puree to the pitcher.

Stir gently to combine all ingredients.

Fill four tall glasses (such as Collins glasses) with ice.

Divide the watermelon mojito mixture amongst the glasses.

Garnish each glass with a lime slice and a sprig of mint.

Serve immediately.