Heat grill to high; oil grates. Place 4 tuna steaks (each 3/4 inch thick and about 6 ounces) in a shallow dish or resealable plastic bag. Add Japanese Marinade; turn to coat. Cover; marinate 15 to 30 minutes in refrigerator, turning fish occasionally.
Just before grilling, remove from refrigerator; remove tuna from marinade. Season with salt and pepper. Grill fish until charred on the outside but still pink inside, 2 to 5 minutes per side.