Preheat your oven to 500°F (260°C). Prepare a 9x13-inch baking dish or baking sheet by coating it with nonstick cooking spray or lining it with parchment paper. Have the prepared baking dish ready before mixing the meatball ingredients.
In a large mixing bowl, combine the ground chicken, panko breadcrumbs, drained crushed pineapple, milk, reduced-sodium soy sauce, sliced green onions, minced garlic, grated ginger, and black pepper. Mix gently until all ingredients are evenly distributed. Make sure the pineapple is thoroughly drained to prevent the mixture from becoming too wet.
Shape the chicken mixture into approximately 20–21 golf-ball-sized meatballs. Arrange them evenly in the prepared baking dish or on the baking sheet. Keep the meatballs similar in size to ensure even cooking, and avoid packing the mixture too tightly.
Place the meatballs in the preheated oven and bake for approximately 15 minutes, or until fully cooked. Check the center of the meatballs with a meat thermometer to ensure they reach an internal temperature of 165°F (74°C).
While the meatballs are baking, prepare the pineapple teriyaki sauce. In a large skillet or saucepan over medium heat, combine the reduced-sodium soy sauce, brown sugar, 1/3 cup water, reserved pineapple juice, minced garlic, and grated ginger. Whisk until the brown sugar dissolves completely.
In a small container, mix the cornstarch with 1 tablespoon of water until smooth. Slowly pour the cornstarch slurry into the teriyaki sauce while stirring. Allow the sauce to simmer for approximately 5 minutes, stirring frequently, until it thickens and develops a glossy consistency.
Transfer the fully cooked chicken meatballs into the saucepan with the pineapple teriyaki sauce. Gently stir to coat each meatball evenly. Allow the meatballs to simmer in the sauce for a few minutes so the flavors combine and the sauce adheres to the meatballs.
Serve the pineapple teriyaki chicken meatballs warm over cooked white rice, spooning additional teriyaki sauce over the top. Alternatively, serve the meatballs on their own. Each serving contains approximately four meatballs with a portion of the sauce.