Prep the Chicken: Place the boneless, skinless chicken thighs in a bowl or a gallon-sized Ziploc bag. Set aside.
Make the Marinade: In a medium mixing bowl, combine the soy sauce, brown sugar, pineapple juice, chopped garlic, and minced ginger. Stir until the sugar is fully dissolved.
Marinate the Chicken: Pour the marinade over the chicken, pressing out as much air as possible before sealing the bag (or tightly covering the bowl). Refrigerate for at least 4 hours, preferably overnight.
Simmer the Marinade: After marinating, remove the chicken from the bag and set aside. Do not discard the marinade. In a medium saucepan, bring the marinade to a simmer over medium heat. Allow it to simmer for 5 to 10 minutes.
Grill the Chicken: Preheat the grill. Place the chicken on the grill and brush some of the hot marinade over the top. Be careful not to return the brush to the marinade container after using it on the chicken to avoid contamination.
Cook Thoroughly: Grill the chicken until it reaches an internal temperature of 165°F (75°C).
Serve: Transfer the cooked chicken to a baking dish and pour the remaining hot teriyaki marinade over it. Serve immediately.
Prep Time: 15 mins | Total Time: 24 hours | Servings: 4-6