BACKGROUND: Bullying is a public health risk with rates amongst pupils in mainstream school estimated to be about 20-30%. This increases to approximately 25-69% amongst pupils with special educational needs and disabilities (SEND). Combined bullying data from two large studies of children and young people from 144 countries found that the greatest risk factor to becoming a victim of bullying was being 'different' to one's peers. These differences included factors such as physical appearance, physical disability or learning disability. Yet there are currently no evidence-based anti-bullying programmes designed specifically for pupils in special schools, and therefore no randomised controlled trials. This study adapted KiVa - an established and evidence-based Finnish anti-bullying programme of ten, 1.5 h lessons that can be embedded into a school curriculum - into KiVa-SEND, by adjusting the language, activities and teaching delivery. METHODS: A two-arm feasibility cluster interventional randomised controlled trial with a 1:1 blocked randomisation allocation ratio of schools and an embedded process evaluation. Data will be collected at baseline and at a 12-month follow-up. Eight UK special education schools will participate with between 128 and 384 pupils and between 16 and 96 teachers completing the data questionnaires. A further three to six teachers and up to 10 pupils will participate in the process evaluation interviews or Talking Mats. Talking Mats is a visual tool to support individuals who struggle with communication, to express their thoughts and emotions. Four schools will be allocated to implement KiVa-SEND across the academic year 2025/26 in addition to anti-bullying practice as usual, and four schools will continue with usual practice alone. Primary outcomes will be feasibility outcomes on the topics of recruitment and retention, adherence, staff surveys, pupil surveys and pupil attendance. Secondary outcomes will focus on pupil survey data, teacher survey data, and the differences between KiVa-SEND and the schools' current anti-bullying programmes. The process evaluation will focus on the topics of recruitment and retention, implementation and adherence of the adapted KiVa programme, engagement and acceptability of/to pupils and staff, and suitability of the outcome measures. DISCUSSION: This feasibility cluster randomised controlled trial with embedded process evaluation will evaluate the feasibility of delivering KiVa-SEND within a variety of UK special schools, the acceptance of the materials, and the suitability of the outcome measures, for pupils aged 7-14 with a range of primary educational needs and learning disabilities. This will inform the feasibility to later conduct a definitive randomised controlled trial of the effectiveness of KiVa-SEND. TRIAL REGISTRATION: ISRCTN, ISRCTN15516577. Registered 31 March 2025 before any data collection, https://www.isrctn.com/ISRCTN15516577.