Use a sharp knife to cut the watermelon into 1-inch thick slices, ensuring they are uniform for even grilling. Arrange the slices on a plate or baking sheet.
Preheat the grill to medium-high heat. Brush both sides of the watermelon slices lightly with olive oil using a basting brush to prevent sticking and promote caramelization.
Place the watermelon slices on the hot grill. Cook for approximately 2-3 minutes per side, flipping carefully with tongs, until grill marks appear and edges begin to char with a smoky appearance.
Remove the slices from the grill and transfer to a serving plate. Optionally, sprinkle with a pinch of salt for enhanced flavor contrast.
Let the grilled watermelon rest for 1-2 minutes, then serve immediately to enjoy the warm, smoky, and juicy texture with a slight caramelized flavor on the edges.