Detach the meat from the rotisserie chicken, discarding the skin and bones. Shred or chop the meat into uniform bite-sized portions.
In a large mixing bowl, whisk together the mayonnaise, lemon juice, poultry seasoning, and salt until smooth and well incorporated.
Add the shredded chicken, diced celery, and chopped onion into the bowl. Fold the ingredients gently until every component is thoroughly coated in the dressing.
Cover and refrigerate for 30 minutes to allow the flavors to meld.