In a bowl, add drained tuna, Greek yogurt, Dijon mustard, lemon juice, celery, red onion, salt, and black pepper. Use a fork to mix well.
Lay out each tortilla on a flat surface. Place chopped lettuce in the center of each tortilla, then evenly distribute the tuna mixture over the lettuce.
Fold in the sides and roll the tortilla tightly into a wrap. Slice in half if desired and serve immediately.