BACKGROUND Primary psoas abscess is a rare disease characterised by a purulent collection in the thickness of the psoas muscle. Its incidence and etiopathogenesis are not well defined. It is mainly encountered in developing countries in weakened conditions. The involvement of an immunocompetent subject is exceptional. We report a case of primary psoas abscess in an immunocompetent patient in Togo. METHODS A 67-year-old retired teacher with no previous medical history was seen in the emergency room with acute and severe abdominal pain. The clinical examination revealed an infectious syndrome and a painful abdominal mass in the right iliac fossa. CT scan showed a heterogeneous fluid collection in the right psoas muscle. Surgical drainage was indicated and performed. The patient was treated postoperatively with dual antibiotics with a favourable outcome. No germs were isolated from the collection. Rare infectious disease, its real incidence is unknown. The most common germ is staphylococcus aureus. Its etiopathogenesis is also poorly understood. Immune deficiency is a predisposing factor for the condition. Diagnosis of the disease is difficult due to a lack of specificity of the signs encountered. Imaging is the key to positive diagnosis with a high sensitivity of CT scan. Treatment is medical-surgical with drainage and antibiotic therapy. CONCLUSIONS Primary psoas abscess is a potentially serious disease requiring early diagnosis and management.