Improving student achievement through teacher collaboration networks is a current focus of schools in many countries. Yet, empirical evidence on the relationship between teacher networks and student achievement and mechanisms that may explain this relationship is limited. This study examined the relationship between teacher networks and student achievement and the mediating role of teachers' collective efficacy beliefs. Data were collected from 53 Dutch elementary schools. Using social network analysis and multiple regression we analyzed teacher survey and student achievement data. Findings indicate that well-connected teacher networks were associated with strong teacher collective efficacy, which in turn supported student achievement. (Contains 5 tables and 1 figure.)