Make the Marinade In a medium bowl, whisk together soy sauce, brown sugar, pineapple juice, garlic, and ginger until sugar is mostly dissolved.
Marinate the Chicken Place chicken thighs in a large zip-top bag or shallow dish. Pour marinade over chicken, seal, and refrigerate for at least 4 hours, preferably overnight for best flavor.
Preheat Grill Heat grill to medium-high heat. Lightly oil the grates to prevent sticking.
Grill the Chicken Remove chicken from marinade (shake off excess) and grill for 5–7 minutes per side, or until nicely charred and internal temp reaches 165°F (74°C).
Optional: Simmer Extra Sauce If desired, bring leftover marinade to a boil in a small saucepan and simmer for 5–7 minutes to use as a glaze or drizzle. (Make sure it reaches a full boil to ensure safety!)
Serve Serve hot with steamed rice, grilled pineapple, or over a fresh green salad for a lighter option.
