Neisseria meningitidis is a gram-negative diplococcus often associated with invasive infections such as meningitis and fulminant sepsis. primary meningococcal septic arthritis (PMSA) is extremely rare. Diagnosis should be prompt and is based on isolation of the germ from synovial fluid or blood culture. This observation reports the case of a 6 -year- old girl who presented to the pediatric emergency room with a painful right knee associated with fever and purpuric spots. The neurological examination was strictly normal with no signs of meningeal involvement. Arthrotomy and drainage were performed. The cytobacteriological examination of the synovial fluid allowed the isolation of a strain of N. meningitidis sensitive to ceftriaxone on culture media. Intravenous antibiotic therapy was then started. The patient improved after adequate treatment. Conclusion: This observation illustrates an unusual presentation of Neisseria meningitidis infections. Prompt bacteriological diagnosis and early treatment combining antibiotics and joint drainage prevented complications in this patient.