Links between mother‐infant affective matching and attachment security are well‐documented, but research on other types of behavioral matching and attachment security are lacking, as are studies that examine these constructs later in children's development. We examine language style matching (LSM) between mothers and their school‐aged children (N = 68), using interviews with each dyad member. As predicted, regressions revealed that higher mother‐child relational LSM was associated with greater child attachment security (operationalized as high security, low dismissal), and that higher LSM predicted smaller increases in children's electrodermal response to a relational probe 1.5 years later. Further, mother‐child relational LSM was a mediator in the indirect path between children's attachment security and children's reactivity. We discuss the potential utility of LSM as a measure of relationship quality and future studies that could refine our understanding of parent‐child language matching.