Given the importance of math in today’s society, it is critical that children who are at risk for math difficulty are identified early. We developed and validated a prekindergarten math subtest (i.e., CIRCLE Progress Monitoring [CPM] Math Subtest). This teacher-completed measure evaluates domains considered important for later math development. Evaluation of validity was undertaken in a longitudinal sample of 383 children (Mage = 4.9 years), and a follow-up sample of 3,691 children (Mage = 4.4 years). The measure demonstrated adequate psychometric properties, including strong internal consistency reliability (α = .94) and test–retest reliability (r = .78). Evaluation of concurrent and predictive validity demonstrated scores on the CPM Math Subtest were correlated with scores on other assessments at high levels (rs from .55–.65). Exploratory and confirmatory factor analyses revealed that the subtest conforms to a well-defined five-factor model that parallels areas considered to be important in the math literature. (author abstract)