Cook pasta according to package directions, omitting salt and fat.
While pasta cooks, coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add chicken, green pepper, and onion; saute until chicken is browned and vegetables are crisp-tender. Stir in tomato sauce, water, and 1/4 teaspoon pepper. Reduce heat, and simmer, uncovered, 5 minutes, stirring often.
Place 3/4 cup drained pasta on each of 4 plates; top each serving with 1 cup chicken mixture.