Background: Assessment for learning has been identified as an effective strategy to help children learn more effectively. Developing children to master basic movement skills in primary school requires formative assessments to inform instruction and learning. This study reports the rationale and methods for an assessment-based intervention that emphasizes fun, mastery and support (A + FMS) designed to improve fundamental movement skill (FMS) proficiency of primary schoolchildren. Methods/design: Utilizing a cluster randomized controlled trial, the A + FMS intervention was designed to improve FMS proficiency of Hong Kong Chinese schoolchildren. A target sample of 282 students or more from 10 Grade 3 classes (from five schools) will be recruited and randomly assigned into an experimental group or a wait-list control group. Competence motivation theory provided a framework for the intervention that emphasizes fun activities to develop basic fundamentals, improving mastery of movement, and providing support for teaching and learning skills. Primary outcome measures are the raw scores of six objectively measured FMS (i.e., jump, hop, skip, dribble, catch, and overhand throw). Secondary outcomes include self-reported measures: enjoyment in physical education, perceived physical competence, perceived skill competence, and perceived social support. Teachers in the experimental group are required to attend a six-h training workshop and integrate 550 min of assessment for learning strategies into their physical education lessons. Resources such as videos, skills checklists, and equipment will also be provided to support children to accumulate extra learning and practice time after school. The rate of changes in primary and secondary outcomes across the experimental and control groups will be compared to determine the effectiveness of the program. Discussion: The A + FMS is an innovative school-based intervention targeting improvements in movement mastery by supporting physical