Purpose: Primary meningococcal arthritis (PMA) represents an uncommon clinical presentation of meningococcal infection, mainly reported among young people. Herein, a case of PMA of the knee in an elderly patient is described.Case presentation: On January 2022, an 87-year-old patient arrived at hospital with continuous fever persisting for three days and a picture of joint pain, swelling, redness, and warmth of her left knee. An arthrocentesis was promptly performed and the inoculated synovial uid turned positive with numerous Gram-negative diplococci at the microscopic examination. The BioFire® FilmArray®Meningitis/Encephalitis (ME) Panel (bioMérieux) quickly con rmed the presence of Neisseria meningitidis. The meningococcal isolate belonged to the serogroup B (MenB), Sequence type (ST)-162/ clonal complex (cc)162.Two grams of ceftriaxone twice a day were administered for 21 days; than cefditoren pivoxil 400 mg twice a day for further 6 weeks after discharge.