Introduction Adults globally are experiencing increases in diseases attributable to overnutrition, yet child undernutrition persists at high levels. This "double burden of malnutrition" increases the risk of nutrition-related non-communicable diseases in mother and child. Evidence-based interventions promote optimal maternal weight or child growth, but gaps remain in implementing them as integrated, sustainable, and equity-focused solutions. Methods and analysis K'ASLEM is an individually randomized, parallel-group Hybrid Type 1 effectiveness-implementation trial in Indigenous Maya communities in Guatemala with high levels of the double burden of malnutrition. We will enroll 766 pregnant women aged [≥]16 years at less than 28 weeks' gestation, allocated 1:1 to intervention or comparator. The intervention integrates (a) food supplementation for pregnant and postpartum mothers and their infants and (b) behavioral counseling to optimize maternal weight and promote healthy nutrition, physical activity, and infant feeding practices. The comparator is enhanced usual care. Mother-child dyads will be followed until 12 months after birth. Co-primary outcomes are maternal weight and child length-for-age z-score at 12 months. Secondary outcomes are child stunting, child development, maternal overweight or obesity, and maternal hemoglobin. We will collect quantitative and qualitative implementation data, use Implementation Mapping to identify factors shaping implementation, and conduct an economic evaluation of costs and cost-effectiveness. Ethics and dissemination Ethics approval was obtained from the Institutional Review Boards of Mass General Brigham and Maya Health Alliance and the Institute of Nutrition of Central America and Panama Research Ethics Committee. Results will be published in peer-reviewed, open-access journals.