Purpose: Teacher content knowledge (CK) and its contribution to student achievement (SA) are understudied in physical education, especially concerning the examination of the effectiveness of professional development (PD) programs using direct measures of teachers' CK and SA. To make progress in this research area, this study investigated the contribution of a content-focused PD program to teachers' CK and SA in basketball, using direct measures thereof. Methods: A quasi-experimental design was utilized to examine the contribution of a PD program. The authors measured the CK of 52 elementary classroom teachers and their fifth or sixth grade students' (n = 913) achievement in basketball before and after the PD program. The data were analyzed using unilevel and multilevel regression analyses. Results: Teachers who participated in the PD program exhibited higher learning gains in their CK; their students also exhibited higher learning gains. Discussion and Conclusion: The study findings suggest that PD programs focused on enhancing teachers' CK might also support SA.