Introduction Depression is a common disorder among older people living in residential aged care facilities. Several trials have demonstrated the effectiveness of behavioural therapies in treating depressive symptoms in older adults living in the community and in residential aged care. Behavioural Activation is demonstrably effective even when delivered by non-specialists (staff without formal psychological training), although strategies for adapting its use in residential aged care facilities are yet to be explored. This study will determine whether training residential care staff in the use of a structured Behavioural Activation programme is more effective at decreasing depressive symptoms among older residents than internet-based training about depression recognition and management alone. Method and analysis The behavioural activation in nursing homes to treat depression (BAN-Dep) trial is a pragmatic two-arm parallel clustered randomised controlled trial. It will recruit 666 residents aged 60 or older from 100 residential aged care facilities, which will be randomly assigned to the Behavioural Activation or control intervention. Staff in both treatment groups will be encouraged to complete the Beyondblue Professional Education to Aged Care e-learning programme to improve their recognition of and ability to respond to depression in older adults. Selected staff from intervention facilities will undergo additional training to deliver an 8-module Behavioural Activation programme to residents with subthreshold symptoms of depression-they will receive ongoing Mental support from trained Behavioural Activation therapists. Outcome measures will be collected by blind research officer at baseline and after 3, 6 and 12 months. The Patient Health Questionnaire-9 is the primary outcome measure of the study. Ethics and dissemination The trial will comply with the principles of the Declaration of Helsinki for Human Rights and is overseen by the University of Western Australia (reference RA/4/20/4234) and Melbourne Health (reference number HREC/18/MH/47) Ethics Committees. The results of this research project will be disseminated through publications and/or presentations in a variety of media to health professionals, academics, clinicians and the public. Only de-identified group data will be presented. Trial registration ACTRN12618000634279.