Mix tuna, mayo, and a couple of pinches of salt in a bowl. Season rice lightly with salt.

Wet hands with water. Take about 3/4 cup rice in one hand, press and form into a triangle shape.

Make a well in the middle of the rice ball and put in tuna mixture. Close the well with 1/4 cup rice.

Reshape and wrap with a sheet of roasted seaweed. Repeat to make another three Onigiri.