BACKGROUND: Psychological interventions which adopt an explicitly interpersonal approach are a recent development in the treatment of distressing voices. AVATAR therapy is one such approach which creates a direct dialogue between a voice-hearer and a computerised representation of their persecutory voice (the avatar) through which the person may be supported to gain a sense of greater power and control. The main objective of the trial is to test the clinical efficacy of this therapy to reduce the frequency and severity of auditory verbal hallucinations (AVH). Secondary objectives of the study are to explore explanatory mechanisms of action and potential moderators, to carry out a qualitative evaluation of participants' experience and to conduct an economic evaluation. METHODS/DESIGN: The AVATAR randomised clinical trial will independently randomise 142 participants to receive either 7 sessions of AVATAR therapy or supportive counselling (SC). The study population will be individuals with schizophrenia spectrum and other psychotic disorders who report hearing persistent distressing voices, for more than 12 months, which are unresponsive or only partially responsive to antipsychotic medication. The main hypotheses are that, compared to SC, AVATAR therapy will reduce the frequency and severity of AVH and will also reduce the reported omnipotence and malevolence of these voices. Assessments will occur at 0 weeks (baseline), 12 weeks (post-intervention) and 24 weeks (follow-up), and will be carried out by blinded assessors. Both interventions will be delivered in a community-based mental health centre. Therapy competence and adherence will be monitored in both groups. Statistical analysis will follow the intention-to-treat principle and data will be analysed using a mixed (random) effects model at each post treatment time point separately. A formal mediation and moderator analysis using contemporary causal inference methods will be conducted as a secondary analysis. The trial is funded by the Welcome Trust (WT). DISCUSSION: AVATAR therapy showed promising effects in a pilot study, but the efficacy of the approach needs to be examined in a larger randomised clinical trial before wider dissemination and implementation in mental health services. TRIAL REGISTRATION: Current Controlled Trials ISRCTN: 65314790 , registration date: 27 March 2013.