Gather the ingredients.

Prick tuna steaks all over with a fork and place in a shallow glass baking dish.

Whisk together oil, soy sauce, lemon juice, Dijon mustard, lemon peel, and garlic in a bowl; pour over tuna steaks. Cover the dish with plastic wrap and refrigerate for 1 to 3 hours.

Preheat the grill to medium heat and lightly oil the grate. Remove tuna from marinade; shake excess marinade from steaks and transfer to a plate.

Pour marinade into a small saucepan; bring to a boil. Reduce heat to medium-low and simmer marinade for 10 minutes.

Cook tuna on the preheated grill, basting with boiled marinade, until cooked through, 5 to 6 minutes per side. Serve with lemon wedges.