Rinse the sushi rice under cold water until the water runs clear, then cook according to the package instructions.
While the rice is cooking, mix the drained tuna, mayonnaise, Sriracha sauce, and salt in a bowl. Adjust the Sriracha and salt to taste.
Once the rice is cooked, let it cool slightly until it's safe to handle. Wet your hands to prevent the rice from sticking, then take a handful of rice and flatten it into a small disk in the palm of your hand.
Scoop a spoonful of the spicy tuna mixture onto the center of the rice disk. Fold the rice over the filling and shape it into a ball or a triangle, pressing firmly to ensure it holds together. Wrap a strip of nori around the onigiri.
Serve the onigiri with soy sauce for dipping, if desired.