Background <p>Child overweight and obesity are major public health challenges associated with poor physical, psychological, and social health through life. This protocol describes the study design for the Bloom Study, an evaluation of a theory- and evidence-based intervention addressing infancy and toddlerhood (age 0–3 years) to promote healthy weight development among children in Denmark.</p> Methods <p>The Bloom intervention is evaluated through an open-label, two-arm, cluster-randomized wait-list control trial, with 22 municipalities randomly allocated to 11 intervention and 11 control municipalities. All first-time families within one year are invited (approximately 3200 families). The intervention was developed using the Intervention Mapping protocol and a thorough co-creation process. It is delivered through the Danish community health nursing system and includes four main components: (1) Home visits, (2) Proactive telephone consultations, (3) Parent groups for both parents, and (4) A video library. The intervention begins during pregnancy (between weeks 22–24) and continues until the child is 30 months. The primary outcome measure is the BMI z-score at 36 months of age. Secondary outcomes relate to the key determinants of child weight development targeted by the Bloom intervention, including food and meals, sleep, movement and motor skills, screen use, and family sense of security. A comprehensive mixed methods process evaluation and a health economic evaluation will also be conducted.</p> Discussion <p>The protocol outlines the evaluation of a theory- and evidence-based intervention aimed at promoting healthy weight development in infants and toddlers in Denmark. Overall, the study has the potential to inform future health promotion initiatives at the municipal level, supporting healthy weight development during early childhood.</p> Trial registration <p>ClinicalTrials.gov ID: NCT06705400. Registered November 21, 2024.</p>