Preheat the oven to 400°F (200°C) and line a large sheet pan with parchment paper or lightly oil it.
In a large mixing bowl, combine the cubed chicken, chopped red bell pepper, chopped red onion, and broccoli florets; add the olive oil, salt, and garlic powder and toss until everything is evenly coated.
Spread the chicken, vegetables, and drained pineapple chunks in a single layer on the prepared sheet pan, leaving a little space between pieces for even roasting.
Bake for 15–18 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and vegetables are tender-crisp.
Remove the pan from the oven, drizzle the teriyaki sauce over the chicken, pineapple, and vegetables, then toss gently with tongs on the pan to coat everything evenly.
Top with the chopped green onions and serve.