Heat grill to medium-high. In a small bowl, combine the pineapple, jalapeño, lime juice, and ¼ teaspoon salt. Set aside.
Form the beef into four ½-inch-thick patties and season with ½ teaspoon each salt and pepper.
Grill the patties (do not press or flatten), 3 to 4 minutes per side for medium-rare.
Place the burgers on the buns and top with the pineapple salsa.