In 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.