Preheat an outdoor grill for medium-high heat and lightly oil grate.
Stir together grapes, capers, shallot, parsley, and olive oil in a bowl; season to taste with salt and pepper, and set aside. Place tuna steaks onto a plate, and brush with lemon juice. Season with salt and pepper to taste.
Cook tuna steaks on preheated grill until cooked to desired degree of doneness, 2 to 3 minutes per side for medium-rare. Serve with the grape and caper salsa.