Mathematical Knowledge for Teaching (MKT) is widely recognized as being beneficial for teachers. Despite the contributions of school psychologists to mathematics education, the MKT possessed by school psychologists has not yet been considered. The current study explores the nature of school psychologists’ (n = 154) MKT pertaining to elementary number and operations. A number of factors were shown to influence school psychologists’ MKT including age, self-efficacy, science-based backgrounds, and mathematical education. Moreover, school psychologists appear to be slightly more mathematically informed in number and operations in comparison to their teacher counterparts. Future research may benefit from evaluating more areas of MKT across a greater number of mathematics domains. Continued research in this area has the potential to ensure that school psychologists are the most effective resource that they can be within schools, thereby improving mathematics education.