Background: Studies have examined relationships between teacher characteristics and student achievement in courses such as math and science. This study is among the first to examine effects of teacher characteristics on student knowledge in a health course. Methods: Student (N = 6143) pretest and posttest data were linked to teacher (N = 67) data. Changes in student knowledge scores from pre- to postcourse were explored using mixed-effects linear models. Teacher characteristics included professional development (PD) attendance, having a dedicated classroom, certification type, educational background, years' experience, and athletic coaching status. Results: Teacher characteristics associated with greater student knowledge gains included: being certified to teach health versus not certified (p < 0.001), having a dedicated classroom versus no classroom (p = 0.017), and for middle school teachers, having attended =3 PD sessions versus =2 (p = 0.023). Less knowledge gain was associated with teachers that coached versus noncoaches (p = 0.040) and having a health degree versus no health degree (p = 0.049). "Post hoc" analyses revealed the negative effect of health degree was only significant among coaches (p = 0.026). Conclusions: Findings suggest opportunities for maximizing student knowledge gains through tailored selection of health teachers and provision of appropriate teaching support.