Background Many people with intellectual disability present with challenging behaviour which often has serious consequences such as the prescription of long term medication, in-patient admissions and disruption of normal daily activities. Small scale studies of Positive Behaviour Support (PBS) delivered by paid carers suggest that it reduces challenging behaviour and costs of care and improves quality of life. This study aims to investigate whether professionals training in the delivery of PBS as part of routine practice is clinically and cost effective compared to treatment as usual in community intellectual disability services. Method The study is a multi-centre cluster randomised controlled trial involving community intellectual disability services in England and service users with mild to severe intellectual disability and challenging behaviour. The teams will be randomly allocated into one of two conditions, either training and support to deliver PBS or treatment as usual. We will carry out assessments of challenging behaviour, use of services, quality of life, mental health, and family and paid carer burden at six and 12 months. We will monitor treatment fidelity and we will interview a sample of paid and family carers, service users, staff and managers about what they think of the treatment and how best we can deliver it in routine care. The main outcome is reduction in challenging behaviour at one year after randomisation. We will also carry out a health economic evaluation to examine the costs and consequences of staff training in PBS. Discussion The study findings will have significant implications for the delivery of PBS in community based services with the potential for reducing inpatient admissions and out-of-area placements for adults with intellectual disability and challenging behaviour.