Dysregulation in children's physiological stress systems is a key process linking early adversity to poor health and psychopathology. Thus, interventions that improve children's stress physiology may help prevent deleterious health outcomes. Reminiscing and Emotion Training (RET) is a brief relational intervention designed to improve maternal caregiving support by enhancing maltreating mothers’ capacity to reminisce with their young children. This study evaluated associations between maltreatment, intimate partner violence, and the RET intervention with changes in children's diurnal cortisol regulation across the 1 year following the intervention, and the extent to which improvements in maternal elaborative reminiscing differed between intervention groups and mediated change in children's physiological functioning. Participants were 237 children (aged 36 to 86 months) and their mothers. Results indicated that the RET intervention was associated with significant positive change in elaborative reminiscing, which was sustained over time. Mothers’ elaboration immediately after the intervention served as a mediator of RET's effects on improvements in children's diurnal cortisol regulation (steeper diurnal slopes) from baseline to 1 year following intervention. This suggests RET is effective in facilitating physiological regulation among maltreated children.