In a medium bowl, combine drained canned tuna with mayonnaise, finely diced celery, and chopped red onion. Mix thoroughly using a fork until well integrated, then season with salt and pepper.

Lay out the nori sheets flat on a clean workspace with the shiny side facing down. Cut them in half if desired.

Spoon a generous amount of tuna salad mixture onto the bottom third of each nori sheet, spreading it evenly.

Starting from the edge with filling, roll the nori tightly towards the naked edge, wetting the edge slightly to seal.

Use a sharp knife to slice each wrap into bite-sized pieces and serve immediately.