In a medium-sized mixing bowl, whisk together pineapple juice, soy sauce, brown sugar, ketchup, rice vinegar, minced garlic, grated ginger, sesame oil, and black pepper until the brown sugar dissolves.

Place chicken thighs or breasts in a resealable bag or shallow dish and pour half of the prepared marinade over. Seal and refrigerate for at least 1 hour, ideally overnight.

Preheat grill to medium-high heat, around 375°F (190°C). Remove chicken from marinade and discard the used marinade. Oil grill grates to prevent sticking.

Grill chicken for approximately 6-7 minutes per side, flipping once. Baste with reserved marinade during the last few minutes. Check for doneness with a meat thermometer (165°F / 75°C).

Transfer grilled chicken to a cutting board and let rest for 3-5 minutes. Slice and serve over a bed of white rice, garnishing with green onions and optional grilled pineapple slices.