Veterans with posttraumatic stress disorder (PTSD) have poorer treatment responses than the general public, which suggests that alternative treatment approaches may be required to support veterans who do not benefit from standard evidence-based approaches. Psilocybin-assisted psychotherapy (PAP) involves administration of psilocybin in a supportive therapeutic setting. PAP has been evidenced to be safe and feasible in a range of recent studies supporting individuals with mental health difficulties including depression, obsessive compulsive disorder, and anxiety. This protocol paper outlines a study aiming to explore the safety and feasibility of PAP for veterans (n = 8) with PTSD. Participants will be offered two dosing sessions of 25 mg of psilocybin, followed by Cognitive Processing Therapy. Eligible participants are UK military veterans who meet criteria for current PTSD and who have completed thorough physical and psychological assessment. The study is subject to ethical approval by the Health Research Authority and Medicines and Healthcare products Regulatory Agency combined review.