We report the first case, to our knowledge, of xanthogranulomatous pyelonephritis (XGP) with psoas abscess occurring during pregnancy. A 37-year-old woman in the third trimester of pregnancy presented with low back pain and a fever. From sonographic features, a multidisciplinary team decided to perform computed tomography of the abdomen with contrast agent injection, which strongly suggested diffuse XGP of the left kidney with a psoas abscess. Cesarean section at 32 weeks was followed by extended nephrectomy. Pathological examination of the operative specimen confirmed the diagnosis. Outcomes were favorable in the mother and baby. The diagnosis and treatment of XGP during pregnancy are discussed.