This research study investigates how elementary teachers' mathematical knowledge for teaching (MKT) relates to their mathematical self-efficacy (MSE) and whether the MKT can predict the MSE. Researchers administered the MSE scale and the MKT test to 86 in-service elementary mathematics teachers. Two-step regression analyses results indicated that MKT and its proxy measures (teaching experience, training type, and hours) were significant predictors of elementary mathematics teachers' MSE level; however, the MKT was the strongest. Considering these results, the research recommends aligning the MKT and the MSE when building teacher preparation and professional development programs. The implications for teacher preparation and developmental programs are further discussed.