Combine the peaches, onion, jalapeño, lime juice, and sugar in a small bowl. Season with salt and pepper, and let stand at room temperature while you prepare the salmon.
Heat grill to medium-high heat.
Season the salmon fillets on both sides with salt and pepper. Using a clean tea towel, wipe grill grates with olive oil. Grill salmon on each side for 4 minutes. Service topped with peach relish.