Heat oven to 350°F. Lightly grease a 2-quart baking dish.
Drain pineapple thoroughly and pat dry to remove excess moisture.
Whisk sugar and flour in a large bowl, then add pineapple and toss to coat.
Fold in shredded cheddar until evenly distributed. Spread mixture into the dish.
Combine cracker crumbs with melted butter. Scatter the crumb mixture over the top.
Bake 28–32 minutes, until the topping is golden and the edges are bubbling.
Rest 10 minutes before serving so the casserole sets slightly.