Cook spaghetti according to package directions; drain. Meanwhile, trim and discard root end of bok choy. Cut stalks into 1-in. pieces. Coarsely chop leaves.
In a large skillet, heat 1 tablespoon oil over medium-high heat. Add chicken; stir-fry 5-7 minutes or until no longer pink. Remove from pan.
Stir-fry bok choy stalks, celery, peppers and onion in remaining oil 4 minutes. Add bok choy leaves; stir-fry 3-5 minutes longer or until leaves are tender. Stir in teriyaki sauce. Add spaghetti and chicken; heat through, tossing to combine.