Line a baking sheet with a large piece of extra strength aluminum foil. Lay the corned beef in the center of the foil.
Combine the teriyaki sauce, soy sauce, and 1 cup of water in a bowl. Poor the mixture over meat. Close the foil around the meat, folding the ends shut tightly. Bake the meat until very tender, about 3 1/2 hours.
Transfer the meat to a cutting board and slice thinly against the grain. Serve immediately.