Sweet Potato Casserole with Marshmallow Topping and Oat Streusel

The perfect holiday side dish, Sweet Potato Casserole with Marshmallows and Oat Streusel. Easy to make ahead, this three-part casserole combines mashed sweet potatoes, mini marshmallows, and a crunchy oat streusel. A classic addition to any Thanksgiving or holiday table.

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Sweet Potato Casserole with Marshmallows and Oat Streusel in a white baking dish with a gold serving spoon sitting on a white kitchen towel.

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Make Ahead Sweet Potato Casserole with Marshmallows

Sweet potato casserole topped with marshmallows is a long-standing American holiday favorite. The idea of pairing marshmallows with sweet potatoes dates back to an early 20th-century promotion, and it’s been a Thanksgiving staple ever since.

This recipe breaks the casserole into three simple layers so you can prep most of it in advance:

Mashed sweet potatoes
Oat streusel
Mini marshmallows

Each component is straightforward to make and stores well, making this an ideal make-ahead side for busy holiday menus.

Supplies

In addition to basic kitchen tools, you’ll need:

A large pot for boiling the potatoes
A potato masher or sturdy spoon
A 2.5‑quart (approx.) baking dish for the casserole
Containers for storing components if prepping ahead

Ingredients

All ingredients are common pantry items. Use the amounts listed in the Ingredients section of the recipe card below.

Sweet potatoes – medium-sized, about the size that fits comfortably in the palm.
Kosher salt – preferred for cooking; table salt will work in a pinch.
Unsalted butter – one amount for mashed potatoes, plus cold butter for the streusel.
Milk – whole milk gives a rich texture, but any milk will do.
Pumpkin pie spice – or substitute half the amount with ground cinnamon if needed.
Brown sugar – light or dark both work in the streusel.
All-purpose flour – for the streusel.
Rolled/old-fashioned oats – use these for the best texture in the streusel.
Baking powder – a small amount for the streusel.
Mini marshmallows – melt quickly; if only regular marshmallows are available, cut them into smaller pieces.

For a complete ingredient list with quantities, see the recipe card below.

Ingredients for Sweet Potato Casserole with Marshmallows and Oat Streusel laid out on a table with a kitchen towel.

Instructions for How to Make Sweet Potato Casserole with Oat Topping and Marshmallows

Break the recipe into the three parts: sweet potato base, oat streusel, and marshmallows. All three can be prepped up to two days ahead.

Notes on Make Ahead

Prepare and store each layer separately. I recommend storing the mashed sweet potatoes in the baking dish you’ll use to bake the casserole. Keep the streusel chilled in an airtight container and measure the marshmallows into a bag or container for convenience. Remove the sweet potatoes from the refrigerator about 30 minutes before baking so they come closer to room temperature. When ready, top with marshmallows, sprinkle on the streusel, and bake.

The three different parts of Sweet Potato Casserole with Marshmallows and Oat Streusel made ahead of time and ready to be stored before cooking in airtight containers.

For the Sweet Potatoes:

Peel and slice the potatoes into 1‑inch rounds. Place them in a large pot with 1 teaspoon salt and cover with cold water. Bring to a boil, then reduce heat and cook 20–25 minutes until tender. Drain, return the pot to the stove over medium heat for 1–2 minutes to evaporate any excess moisture, then remove from heat. Add 2 tablespoons butter, 1/4 cup milk, and 1 teaspoon pumpkin pie spice, and mash gently until smooth. Use immediately or store in an airtight container in the fridge.

The three steps of cooking the sweet potatoes, uncooked, in water, and cooked.

For the Oat Streusel:

Cut cold butter into marble-sized pieces. In a medium bowl combine 4 tablespoons cold butter, 1/4 cup brown sugar, 1/4 cup flour, 1/4 cup rolled oats, 1/2 teaspoon baking powder, 1/2 teaspoon pumpkin pie spice and a pinch of salt. Use two forks, a pastry cutter, or clean fingers to cut the butter into the dry ingredients until coarse crumbs form, about the size of large peas. Keep the streusel chilled until ready to bake so the butter remains solid.

Steps of making oat streusel, adding the ingredients to a bowl and mixing until a corse crumb in achieved.

Making & Cooking the Casserole:

Preheat the oven to 350ºF (you can bake at 375ºF if needed; baking time will be shorter—watch the marshmallows). Grease a 2½‑quart baking dish and spread the mashed sweet potatoes evenly in the dish. Top with 3 cups mini marshmallows, then sprinkle the oat streusel evenly over the marshmallows. Bake 18–20 minutes, watching closely so the marshmallows don’t scorch. Remove from the oven and let rest for at least 10 minutes before serving to allow the filling and toppings to set.

Hands holding a white baking dish with Sweet Potato Casserole with golden Marshmallows and crunchy Oat Streusel.

Tips, Tricks, & Questions

Why is my casserole watery and how can I thicken it?

A watery casserole usually results from excess moisture. Two common causes:

Serving too soon after baking — let the casserole rest at least 10 minutes to allow it to set.
Insufficiently dried potatoes — after draining, briefly return potatoes to the pot over heat to steam off excess water before mashing.

Can I double it?

Yes—use a larger pan such as a 9×13 for doubled quantities and adjust baking time as needed.

Sweet potatoes vs. yams?

In most U.S. grocery stores the labels “sweet potato” and “yam” are used interchangeably. True yams are different but are rarely encountered in typical American supermarkets.

Sweet Potato Casserole with Marshmallows and Oat Streusel in a white casserole dish with brown marshmallows.

Close up of a scoop missing from Sweet Potato Casserole with Marshmallows and Oat Streusel showing the inside of the casserole.

Need more Thanksgiving and holiday dishes? You might like these:

5 Minute Cranberry Orange Relish
Roasted Carrots with Dill
Instant Pot Mashed Potatoes for a Crowd
Roasted Pear & Pecan Salad
Apple Cranberry Salad with Apple Cider Vinaigrette
All Butter Pie Crust
Sunflower Seed Pie

Yield: 8 servings