Background: Systematically screening for child anxiety problems, and offering and delivering a brief, evidence-based intervention for children who are identified as likely to benefit would minimise common barriers that families experience in accessing treatment. We have developed a short parent-report child anxiety screening questionnaire, and procedures for administering screening questionnaires, sharing screening outcomes with families, and offering and delivering a brief parent-led online intervention (OSI: Online Support and Intervention for child anxiety) through schools. This trial aims to evaluate clinical and health economic outcomes for (1) children (aged 8–9) who screen positive for anxiety problems at baseline (target population) and (2) the wider population of all children in participating classes (total population) in schools randomly allocated to receive identification-to-intervention procedures and usual school practice (‘screening and intervention’), compared to assessment and usual school practice only (‘usual school practice’). Methods: The trial design is a parallel-group, superiority cluster randomised controlled trial, with schools (clusters) randomised to ‘screening and intervention’ or ‘usual school practice’ arms in a 1:1 ratio stratified according to the level of deprivation within the school. We will recruit schools and participants in two phases (a pilot phase (Phase 1) and Phase 2), with progression criteria assessed prior to progressing to Phase 2. In total, the trial will recruit 80 primary/junior schools in England, and 398 children (199 per arm) who screen positive for anxiety problems at baseline (target population). In schools allocated to ‘screening and intervention’: (1) parents/carers will complete a brief parent-report child anxiety screening questionnaire (at baseline) and receive feedback on their child’s screening outcomes (after randomisation), (2) classes will receive a lesson on managing fears and worries and staff will be provided with information about the intervention and (3) parents/carers of children who screen positive for anxiety problems (target population) will be offered OSI. OSI will also be available for any other parents/carers of children in participating classes (total population) who request it. We will collect child-, parent- and teacher-report measures for the target population and total population at baseline (before randomisation), 4 months, 12 months and 24 months post-randomisation. The primary outcome will be the proportion of children who screen positive for anxiety problems at baseline (target population) who screen negative for anxiety problems 12 months post-randomisation. Discussion: This trial will establish if systematic screening for child anxiety problems, sharing screening outcomes with families and delivering a brief parent-led online intervention through schools is effective and cost-effective. Trial registration: ISRCTN registry ISRCTN76119074. Prospectively registered on 4.1.2022.