Pyometra is an easily overlooked disease with nonspecific symptoms; however, a delayed diagnosis can lead to severe complications. An 80-year-old frail woman presented to our hospital with a chief complaint of persistent fever for 10 days. Her blood tests showed an elevated inflammatory response, and computed tomography showed a 10-cm cystic lesion in the pelvic floor compressing the bladder. A catheter was inserted from the vagina into the uterine cavity, resulting in pus drainage and pyometra diagnosis. A pus culture was subsequently performed, which detected , a common cause of respiratory tract infections in cystic fibrosis and bloodstream infections, and. To the best of our knowledge, this is the first report of pyometra caused by . The patient was treated with drainage and piperacillin-tazobactam administration. Pyometra is especially prevalent in older women with impaired activities of daily living and dementia. Although fever, lower abdominal pain, and increased discharge may occur, symptoms are often nonspecific, and half of such cases are asymptomatic. Furthermore, delayed diagnosis can lead to perforation of the uterus and consequent pan-peritonitis. Thus, the diagnosis of pyometra should be considered in older women presenting with unknown fever, and imaging studies and gynecological consultation should be requested promptly.