Preheat the oven to 425 degrees F and line two rimmed baking sheets with foil.
Leaving the peels on, cut the red potatoes into strips about 1/2 x 1/2 x 3-4 inches. (The more consistent the size is, the better the fries bake.) Place the fries on a baking sheet and top with oil and harissa. Toss and coat thoroughly. Spread the fries out over both baking sheets, so that none of them touch.
Bake for 20 minutes, then gently flip and bake another 10-15 minutes, until golden and crisp. Serve warm.