Use a sharp knife to cut the watermelon into 1-inch thick slices on a cutting board. Remove any seeds if present.
Brush both sides of each watermelon slice lightly with olive oil using a pastry brush or paper towel.
Preheat the grill or grill pan over high heat until hot, about 5 minutes. Place the watermelon slices on the grill, positioning them perpendicular to the grates to create appealing grill marks.
Grill the watermelon slices for about 2-3 minutes per side, until the edges develop a deep golden-brown sear and slight charring is visible. Use tongs to carefully flip the slices and repeat on the other side.
Once both sides are charred and the watermelon begins to soften, remove the slices from the grill and place on a serving plate. Let rest for a minute to allow flavors to settle.
Optional: Sprinkle with a pinch of salt before serving to enhance flavor. Serve immediately to enjoy the smoky caramelized exterior with juicy interior.
1 large seedless watermelon