Sweet Potato Casserole with Marshmallows and Oat Streusel

There’s something wonderfully nostalgic about a Sweet Potato Casserole with Marshmallows and Oat Streusel — it’s the kind of dish that smells like home, feels cozy on the inside, and always gathers people around the table. In this recipe, I’ve balanced the silky sweetness of mashed sweet potatoes with warm spices, topped it with a buttery oat streusel for crunch, and finished with a pillow of toasted marshmallows. Expect a comforting, slightly indulgent side that pairs perfectly with roast turkey, glazed ham, or a simple weeknight roast.

Why You’ll Fall for This Sweet Potato Casserole with Marshmallows and Oat Streusel

This casserole strikes a lovely balance between creamy and crunchy. The sweet potatoes are mashed with butter and spices so they’re rich but not cloying, while the oat streusel brings a crunchy counterpoint that makes each bite interesting. The marshmallows add that classic, toasty finish many of us grew up loving, but the oat streusel keeps the dish from feeling overly sweet. It’s familiar, crowd-pleasing, and easy to customize.

What You’ll Need — Ingredients for Sweet Potato Casserole with Marshmallows and Oat Streusel

• Sweet potatoes (3 pounds) — The heart of the dish; choose firm, medium sweet potatoes for the best texture.
• Unsalted butter (1/3 cup) — Adds creaminess and a velvety mouthfeel when mixed into the mash.
• Brown sugar (1/3 cup) — Deepens the natural sweetness and gives a slight caramel note.
• Milk (1/4 cup) — Thins the mash slightly so it’s easy to spread; whole milk gives a richer result.
• Vanilla extract (1 tsp) — Brings a warm, aromatic backdrop to the flavors.
• Ground cinnamon (1/2 tsp) & nutmeg (1/4 tsp) — Classic spices that add warmth without overpowering the sweet potatoes.
• Salt (1/4 tsp) — Essential for balancing sweetness and enhancing all the flavors.
• Mini marshmallows (2 cups) — The nostalgic, gooey topping that toasts up beautifully.
• Rolled oats (1 cup) — The base of the streusel; old-fashioned oats provide good texture.
• All-purpose flour (1/2 cup) — Helps bind the streusel so it crisps up nicely.
• Packed brown sugar (1/2 cup) — Sweetens and caramelizes the streusel.
• Cold unsalted butter (1/3 cup) — Cut into the oat mixture to create crumbly pockets that bake into golden clusters.
• Salt (1/4 tsp for streusel) — Balances the sweet notes of the topping.
• Chopped pecans (1/3 cup, optional) — Add if you want extra crunch and nutty flavor.

Step-by-Step Guide to Making Sweet Potato Casserole with Marshmallows and Oat Streusel

1. Preheat your oven to 375°F (190°C). Scrub and pierce the sweet potatoes, then bake them whole on a sheet until very tender (about 45–60 minutes). If you’re short on time, steam or boil the potatoes until soft.
2. When the potatoes are cool enough to handle, peel and mash the flesh in a large bowl. A ricer or fine masher will give a smoother texture; a hand masher gives a more rustic feel.
3. Stir in butter, brown sugar, milk, vanilla, cinnamon, nutmeg, and salt. Taste and adjust sweetness or spices to suit your preference. The mixture should be creamy and spreadable, not runny.
4. Spread the sweet potato mixture into a greased 9x13-inch baking dish in an even layer, smoothing the top with a spatula so the topping will sit nicely.
5. For the streusel, combine oats, flour, and packed brown sugar in a bowl. Add small cubes of cold butter and use a pastry cutter or your fingertips to work the butter into the dry ingredients until you have coarse crumbs. Fold in chopped pecans if using.
6. Sprinkle about two-thirds of the streusel evenly over the sweet potatoes and bake uncovered for 20 minutes. This short bake warms the filling and allows the streusel to begin setting.
7. Remove from the oven and add mini marshmallows across the surface, then top with the remaining streusel. Return to the oven and bake another 8–12 minutes until the marshmallows are puffed and slightly golden and the streusel is crisp. For an extra-toasty finish, broil for 30–60 seconds while watching closely.
8. Let the casserole rest for 5–10 minutes before serving so the filling firms slightly and slices hold together. Serve warm and enjoy the contrast of creamy, sweet, and crunchy.

Timing & Preparation Details

• Active prep time: about 30 minutes (peeling, mashing, and making streusel).
• Total time (with baked sweet potatoes): around 1 hour 20 minutes — most of that is passive baking time for the potatoes.
• Assembly and final bake: 30–35 minutes (including warming and toasting the marshmallows).
• Resting: 5–10 minutes after baking makes serving neater and more enjoyable.
• Make-ahead tip: Prepare the mashed sweet potatoes and streusel separately, refrigerate, and assemble right before baking. You may need to add a few extra minutes if baking from chilled.

Nutritional Snapshot

Approximate nutrition per serving (serves 8):

• Calories: 420 kcal
• Protein: 4 g
• Carbohydrates: 58 g (Sugar ~28 g)
• Fat: 18 g
• Fiber: 4 g
• Sodium: varies with added salt and butter

These values are estimates and will change based on exact ingredients and portion sizes. For a lighter version, reduce butter and brown sugar in both the filling and streusel, or swap part of the butter for applesauce in the mash.

Frequently Asked Questions

Can I make this casserole ahead of time?

Yes. You can prepare the sweet potato mash and the streusel separately a day ahead. Keep them refrigerated and assemble just before baking. If baking from chilled, add an extra 10–15 minutes to the baking time so the center heats through.

What’s the best way to avoid burnt marshmallows?

To prevent marshmallows from burning, add them partway through the final bake — or sprinkle remaining streusel over the marshmallows to shield them. You can also use the broiler for a very short time at the end for a controlled toasting, watching closely.

Can I make this without marshmallows or with a different topping?

Absolutely. Omit marshmallows and use the full streusel topping for a less sweet version. You can also top with toasted coconut or extra pecans for variety. For a dairy-free option, use a plant-based butter and milk substitute.

There you have it — a cozy, crowd-pleasing Sweet Potato Casserole with Marshmallows and Oat Streusel that’s simple to assemble, full of comforting flavors, and easy to tweak for your family’s tastes. Whether it’s for a holiday table or a comforting weeknight side, this recipe aims to give you that warm, nostalgic feeling with every spoonful. Happy cooking!

Sweet Potato Casserole with Marshmallows and Oat Streusel

A comforting, lightly spiced sweet potato casserole topped with golden marshmallows and a crunchy oat streusel. This dish balances creamy sweet potatoes with a toasted, buttery crumble for a festive side everyone will love.

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Prep Time 30 minutes mins

Cook Time 20 minutes mins

Servings 8

Calories 420 kcal

Ingredients

  

• 3 pounds sweet potatoes
• 1/3 cup unsalted butter
• 1/3 cup brown sugar
• 1/4 cup milk
• 1 teaspoon vanilla extract
• 1/2 teaspoon ground cinnamon
• 1/4 teaspoon ground nutmeg
• 1/4 teaspoon salt
• 2 cups mini marshmallows
• 1 cup rolled oats
• 1/2 cup all-purpose flour
• 1/2 cup packed brown sugar
• 1/3 cup cold unsalted butter
• 1/4 teaspoon salt
• 1/3 cup chopped pecans (optional)

Instructions

 

• Preheat the oven to 375°F (190°C). Scrub the sweet potatoes, pierce them a few times with a fork, and place on a baking sheet. Bake until very tender, about 45–60 minutes depending on size. Alternatively, steam or boil until tender if you prefer faster cooking.
• Once the sweet potatoes are cool enough to handle, peel them and transfer the flesh to a large bowl. Mash with a potato masher or pass through a ricer for an extra-smooth texture.
• Add the butter, brown sugar, milk, vanilla, cinnamon, nutmeg, and salt to the mashed sweet potatoes. Mix until smooth and taste, adjusting seasoning or sweetness if needed.
• Spoon the sweet potato mixture into a 9x13-inch (or similar) baking dish, spreading it into an even layer. Smooth the top with a spatula.
• Make the oat streusel: In a medium bowl, combine rolled oats, flour, packed brown sugar, and salt. Add the cold butter cut into small pieces. Use a pastry cutter or your fingertips to work the butter into the dry ingredients until the mixture resembles coarse crumbs. Stir in the chopped pecans if using.
• Sprinkle about two-thirds of the oat streusel evenly over the sweet potato layer. This helps the potatoes heat through with a crunchy interior under the marshmallows.
• Bake the casserole uncovered at 375°F (190°C) for 20 minutes to warm the filling and set the streusel. Remove from the oven and evenly distribute the mini marshmallows across the top, then sprinkle the remaining streusel over the marshmallows (this keeps the marshmallows from burning too quickly).
• Return the casserole to the oven and bake for an additional 8–12 minutes, or until the marshmallows are puffed and golden and the streusel is crisp. For extra toasting on the marshmallows, place under the broiler for 30–60 seconds — watch very closely to avoid burning.
• Let the casserole rest for 5–10 minutes before serving so the filling firms slightly. Serve warm and enjoy!