Preheat an oven to 450 degrees F (230 degrees C). Grease a baking sheet and line with oil.
Cut sweet potatoes (unpeeled) into 1/2-inch wedges and arrange on the prepared baking sheet in a single layer. Drizzle with oil and season with salt and pepper.
Roast, stirring halfway through, until browned and tender, about 30 minutes. Transfer to a large bowl.
Meanwhile, heat butter in a small skillet over medium heat until melted and golden brown, about 5 minutes. Stir in rosemary and orange zest. Pour over sweet potato wedges and toss to coat.