In a gallon size resealable bag add balsamic vinegar, olive oil, garlic, salt, and fresh ground pepper. Strip the rosemary leaves from the stem and add them to the bag.
Close the bag and shake well to combine, then add chicken and reseal the bag. Make sure the marinade is evenly distributed around the chicken. Marinate overnight for the best flavor.
Grill over medium high heat until juices run clear and temperature registers 160°.