Background: Health education (HE) courses in schools are vital paths for improving teenagers' health literacy. HE and physical education (PE) teachers lead HE courses, and their teaching intentions and competency influence the effectiveness of the courses and the ability to promote students' health literacy. This study attempted to understand HE and PE teachers' health literacy teaching intentions and professional competency and to investigate their relationships. Methods: This study adopted a cross-sectional design. A questionnaire survey was administered to 906 middle school HE and PE teachers in Taiwan by mail, and 545 provided valid data with consent. Results: Participants had a favorable health literacy (47.78/50), positive health literacy teaching beliefs and attitudes, and acceptable efficacy. They intended to implement health literacy instruction within the subsequent year. Teaching beliefs, attitudes, and efficacy were all positively correlated with intentions (all p < 0.001). Demographic variables, health literacy, and teaching beliefs, attitudes, and efficacy explained 33.5% of the variance of teaching intentions. Conclusion: Teaching beliefs, attitudes, and efficacy were crucial predictors of health literacy teaching intentions. To improve students' health literacy, educational authorities and schools should pay attention to HE and PE teachers' intentions and vital factors, thereby enhancing teachers' willingness to perform health literacy instruction.