The aim of this study was to test the efficiency of an extended model of the theory of planned behavior (TPB) in predicting intention to improve oral health behaviors. The participants in this cross-sectional study were 153 first-year medical students (mean age 20.16, 50 males and 103 females) who completed a questionnaire assessing intentions, attitudes, subjective norms, perceived behavioral control, oral health knowledge, and current oral hygiene behaviors. Attitudes toward oral health behaviors and perceived behavioral control contributed to the model for predicting intention, whereas subjective norms did not. Attitudes toward oral health behaviors were slightly more important than perceived behavioral control in predicting intention. Oral health knowledge significantly affected affective and cognitive attitudes, while current behavior was not a significant predictor of intention to improve oral health behavior. The model had a slightly better fit among females than among males, but was similar for home and professional dental health care. Our findings revealed that attitude, perceived behavioral control, and oral health knowledge are predictors of intention to improve oral health behaviors. These findings may help both dentists and dental hygienists in educating patients in oral health and changing patients’ oral hygiene habits.