Following a severe traumatic brain injury (TBI) there is a complex presentation of psychological symptoms which may impact on recovery. Validated treatments addressing these symptoms for this group of people are limited. This article reports on the protocol for a single-centre, two-armed, Phase II Randomised Control Trial (RCT) to address the adjustment process following a severe TBI. Participants will be recruited from Liverpool Brain Injury Rehabilitation Unit and randomly allocated to one of two groups, Acceptance and Commitment Therapy (ACT) or an active control (Befriending). The active treatment group utilises the six core processes of ACT with the intention of increasing participation and psychological flexibility and reducing psychological distress. A number of primary and secondary outcome measures, administered at assessment, post-treatment and 1-month follow-up, will be used to assess clinical outcomes. The publication of the protocol before the trial results are available addresses fidelity criterion (intervention design) for RCTs. This ensures transparency in the RCT and that it meets the guidelines according to the CONSORT statement. The protocol has also been registered on the Australian New Zealand Clinical Trials Registry ACTRN12610000851066.