Children growing up in poverty have a higher likelihood of exposure to multiple forms of adversity that jeopardize their chances of academic success. The current paper identifies school mobility, or changing schools, as 1 such poverty-related risk. Using a sample of low-income, predominantly ethnic-minority children (n = 381) in Chicago, this study tests the hypothesis that repeatedly changing schools during the 5-year period between Head Start (preschool) and third grade is a potent predictor of children's math achievement in fourth grade and that children's cognitive dysregulation serves as a mechanism through which school mobility may negatively affect children's math achievement. Hierarchical linear models controlling for baseline child and family characteristics (including children's early math and dysregulation measured during Head Start) revealed an inverse relation between the number of times low-income children changed schools between preschool and third grade and children's math achievement on state standardized tests in fourth grade. Furthermore, frequently changing schools (3 or 4 school changes over the same time period) was positively associated with teacher-reported cognitive dysregulation in third grade and negatively associated with children's math achievement in fourth grade. Evidence for the role of children's cognitive dysregulation as a partial statistical mediator was found for the relation between frequently changing schools and math achievement, even after accounting for baseline risk. Results are discussed in terms of school policies, practices, and intervention strategies to prevent the disruptive and potentially stressful experiences of school mobility for young, low-income children. (author abstract)