A 70-year-old male patient was admitted to a hospital on March 1, 2024 due to "muscle soreness in his extremities for over a month". Diagnosis consideration: polymyalgia rheumatica (with a high likelihood, the possibility of a tumor needs to be excluded). The patient was treated with methylprednisolone. After discharge from the hospital, the patient’s symptoms worsened due to self-withdrawal of medication (methylprednisolone treatment for 20 days),and developed fever and cough. He then revisited the hospital and was confirmed to have Legionella mar-ssiliensis infection through metagenomic sequencing of bronchoalveolar lavage fluid (nucleic acid of all microorga-nisms were extracted from the specimens and compared in the PMDB database to obtain species information of the suspected pathogenic microorganisms). Subsequently, the patient was treated with a combination of 0.75 g levofloxacin intravenous infusion qd + 0.1 g doxycycline enteric-coated capsules orally bid for anti-infective therapy. The patient’s symptoms, such as cough and muscle pain, improved significantly after anti-inflammatory and anti-infective treatment, and he was discharged on March 18. As the first reported case of Legionella marssiliensis pulmonary infection in China, this case highlights that among the multiple species of Legionella, there is another bacte-rium that can infect the human body and cause disease. This case report is beneficial for improving medical staff’s understanding on Legionella marssiliensis and providing reference for the diagnosis and treatment of future cases of Legionella marssiliensis infection.