Heat grill to medium. Grill the pineapple slices, turning occasionally, until softened, about 3 minutes per side. Transfer to a cutting board. Let stand for at least 2 minutes. Dice the pineapple. Transfer to a medium bowl with the jalapeño, cilantro, onion, and lime juice. Serve warm or at room temperature.
Tip: Try this sweet-tart salsa with any grilled seafood, pork, or chicken.