SummaryIn this systematic review and realist synthesis, we aimed to identify strategies to improve dietary and physical activity behaviors for parents with small children. A systematic literature search was conducted in MEDLINE, Embase, CINAHL, and PsycINFO in July 2021. Included studies had to: (i) target one or both parents with at least one child (0–3 years), (ii) aim to improve diet and physical activity, and (iii) report on diet and physical activity outcomes. Intervention context, delivery, and outcomes were extracted, and behavior change techniques were coded. A program theory was developed, and context‐mechanism‐outcome configurations were identified. In total, 17 interventions reported in 28 studies (19 effectiveness studies; nine protocols) were included. Nine interventions showed small improvements: in diet (n = 5), physical activity (n = 2), or both (n = 2) in mothers. The realist synthesis revealed three strategies to improve health behaviors: (1) using knowledge and role modelling to improve family dynamics, (2) providing various home‐based activities to change home environment, and (3) offering flexible delivery, for example, phone or website‐based to increase social support. Future interventions for parents with small children should consider involving the whole family, focusing on home‐based and practical components, and offering various delivery modes. The protocol for the systematic review and realist synthesis was registered in Research Registry (registration ID: reviewregistry860) March 30th, 2020.