Prepare chicken and vegetables: Cut chicken into bite-sized pieces, dice bell peppers and onion, and drain pineapple chunks.
Combine sauce ingredients: In a bowl, whisk together ketchup, rice vinegar, brown sugar, soy sauce, garlic powder, and ginger.
Add to crockpot: Place chicken, bell peppers, onion, and pineapple into the slow cooker. Pour sauce over the top and stir gently to coat.
Slow cook: Cover and cook on low for 4–5 hours or on high for 2–3 hours until chicken is fully cooked and vegetables are tender.
Thicken sauce: In a small bowl, mix cornstarch with water to create a slurry. Stir into the crockpot and cook for an additional 15–20 minutes to thicken the sauce.
Serve: Spoon sweet and sour chicken over steamed rice or noodles. Garnish with chopped green onions or sesame seeds if desired.