Drain the tuna in a fine-mesh strainer and press gently; blot with paper towels until dry.
Transfer the drained tuna to a medium bowl and flake it with a fork until chunky but broken up.
Add the mayonnaise, diced celery, sliced scallions, lemon juice, Dijon mustard, sweet pickle relish, chopped parsley, salt, and pepper to the bowl.
Stir gently until all ingredients are evenly combined; taste and adjust seasoning if needed.
Use the tuna salad immediately or refrigerate in a covered container for up to three days.