3 cups finely diced seedless watermelon (about 2¼ pounds with the rind)
2 jalapeño peppers, seeded and minced
⅓ cup chopped cilantro (about ½ bunch)
¼ cup lime juice
¼ cup minced red onion (about ½ small)
¼ teaspoon salt, or to taste
Place watermelon, jalapeños, cilantro, lime juice and onion in a medium bowl; stir well to combine.
Season with salt. Serve at room temperature or chilled.
To make ahead: Cover and refrigerate for up to 1 day.