Use a sharp knife and cutting board to cut watermelon into 1-inch thick slices, then transfer to a plate.
Preheat the grill to medium-high heat (around 375-450°F), ensuring the grates are clean and well-oiled.
Place watermelon slices directly on the hot grill grates, allowing them to sit undisturbed for 2-3 minutes until grill marks start to appear and edges are slightly caramelized.
Carefully flip the slices using tongs and continue grilling for another 2-3 minutes, observing the edges turning golden brown with small charred spots and a smoky aroma.
Remove the grilled watermelon from the heat, sprinkle with a pinch of salt if desired, and let cool slightly before serving. The slices should be juicy with crispy, caramelized edges and a smoky scent.