Preheat oven to 425 degrees. Line a large rimmed baking sheet with foil and spray with cooking spray. Set aside.
In a large Ziploc bag (or in a large bowl), combine olive oil, rosemary, garlic, salt and pepper. Add chicken and potatoes; toss to coat.
Spread chicken (skin-side up) and potatoes in a single layer on the prepared baking sheet. Season chicken with additional salt and pepper.
Bake for about 30 minutes (stirring potatoes halfway through), or until chicken reaches an internal temperature of 165 degrees F and the potatoes are tender and crispy. Toss potatoes in pan juices just before serving. Garnish with additional rosemary, if desired.
