In a sealable plastic bag or large dish, whisk together brown sugar, ketchup, soy sauce, rice vinegar, ginger, and garlic. Reserve ½ cup marinade for basting.
Add chicken to the marinade and let it marinate for at least 2 hours or overnight for best flavor.
Preheat grill to medium heat and oil the grates.
Grill marinated chicken for 20-25 minutes, turning every 5 minutes and basting with reserved marinade until internal temperature reaches 165°F.
Let rest before serving with optional toppings like cilantro or green onions.