In a bowl combine the rice vinegar, soy sauce, sesame oil, and brown sugar together. Microwave for 45 second to warm the liquid and whisk to dissolve the sugar.
Reserve 2 tablespoons of the marinade. Using the remaining marinade, marinate the tuna for 30 minutes. While it is marinating heat your grill and brush a little oil on the grill to prevent sticking.
Place the tuna over direct heat and sprinkle the sesame seeds on top. Cook for 2 minutes per side for a nice sear.