Preheat the oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish.
Mix the filling: In a large bowl, combine the sour cream, cream of chicken soup, and poppy seeds. Stir until smooth.
Add chicken: Fold in the shredded chicken until evenly coated.
Transfer to dish: Spread the chicken mixture evenly into the prepared baking dish.
Prepare the topping: In a separate bowl, mix the crushed Ritz crackers with the melted butter.
Top the casserole: Sprinkle the cracker mixture evenly over the top.
Bake: Bake uncovered for 30–35 minutes, or until hot and golden brown.
Rest: Let the casserole rest for 5 minutes before serving.