This study examines the effect of teachers’ participation in mathematics and science professional development (PD) on student achievement in nationally representative settings. We use data from all OECD countries in the 2003 through 2019 cycles of the Trends in International Mathematics and Science Study (TIMSS) and apply student fixed effects to control for unobserved student characteristics and school quality. We find a small negative average effect of PD participation, with negative effects concentrated among high-performing students. We discuss potential explanations of these results and suggest ways PD studies may inform the PD in which teachers typically participate.