Prepare the marinade by combine soy sauce, calamansi, ground black pepper, and sugar in a bowl. Stir until well blended.

Place the tuna in a Ziploc bag. Pour the soy sauce marinade.Let the air out of the bag. Seal. Marinate inside the fridge for at least 3 hours.

Heat-up the grill. Arrange the tuna belly on the grate of the grill. Brush a generous amount marinade mixture on the tuna. Grill for 3 to 5 minutes.

Turn the tuna belly over and grill the opposite side. Baste the top and sides. Continue to grill for another 5 minutes. Continue performing the process until the tuna is completely cooked.

Serve with a dipping sauce composed of soy sauce, calamansi, and chili pepper. Share and enjoy!