With the increasing demand for improved student achievement in mathematics, few studies have been conducted examining teacher efficacy when continuously supported through a collaborative professional learning community (PLC) structure. This study examined teacher efficacy within PLC activities in a middle school. Both the belief that mathematics instruction would result in student learning and the belief to effectively teach mathematics statistically significantly increased after one year of participation with large effects (1.15 and 1.30, respectively). The correlations between belief that mathematics instruction would result in improved student learning and in their ability to effectively teach mathematics were very strong and positive.