Preheat your oven to 400°F (200°C).
Place the chicken, bell peppers, and onion on a large sheet pan.
Drizzle with olive oil and sprinkle fajita seasoning over the top. Toss to coat evenly.
Spread everything into a single layer on the sheet pan.
Bake for 20-25 minutes, stirring halfway through, until the chicken is cooked and vegetables are tender.
Serve warm with tortillas and your favorite toppings.