In a medium mixing bowl, combine your diced grilled chicken, chopped red onion, and chopped celery.
Gently fold in the sliced avocado. Handle it carefully so the pieces stay intact and don't turn into mash.
Pour the ranch dressing over the ingredients. Using a spatula, toss gently until the chicken and vegetables are evenly coated.
Add salt and black pepper to taste. Give it one final light toss to distribute the seasoning.
If you have the time, let the salad rest in the refrigerator for 10 minutes to let the flavors meld. Serve chilled on its own, in a wrap, or over greens.