Place potato in a large pot of water. Cook over a high flame until it reaches a boil, then lower flame to a gently boil and cook for 15 minutes or until the potato pieces are fork tender.

Rinse potatoes in a colander under cool water.

Place the potatoes in a large bowl. Add onion, celery, red pepper, Ranch dressing, and Cajun seasoning mix. Stir well to combine the ingredients.

Cover and refrigerate for at least 2 hours.