Psoas abscess is a rare intramuscular infection of the psoas, or iliopsoas muscle, caused typically by Mycobacterium tuberculosis or Staphylococcus aureus, including methicillin-resistant S aureus (MRSA). It may be attributed to the spread of bacteria by the circulatory (hematogenous) route from infected sites anywhere in the body (primary abscess), or from contact with adjacent organs (secondary abscess).

Primary psoas muscle abscess is more common in persons under age 30 (83% of cases), and is more common in males. Although the etiology of primary psoas abscess is often uncertain, the majority of blood samples (88%) test positive for S aureus. Immunocompromised patients with human immunodeficiency virus (HIV), renal disease, diabetes, intravenous drug use, or alcohol use disorder are at risk for developing psoas abscess.

Secondary psoas muscle abscess is more likely to occur in adults with concurrent illness. The digestive disorders most often associated with secondary abscess are perforated appendicitis, perforated diverticulitis, Crohn disease, colorectal cancer, and pancreatic abscess. Other associated conditions include septic arthritis, suppurative adenitis, endocarditis, urinary tract infection, tuberculous spondylitis, osteomyelitis of the vertebrae, infectious sacroiliitis, and lumbar spondylodiscitis.

Psoas abscess has been reported during pregnancy.