Introduction: The prevalence of septic arthritis (SA) is higher in underdeveloped countries than in developed countries, and the number of hospitalizations of children for this cause is decreasing and the most prevalent age group is between 0 and 4 years of age. Clinical history and detailed physical examination are essential to make an accurate diagnosis, but the neonatal period has certain limitations. Objective: To report a case of septic arthritis that occurred in a newborn admitted to the Neonatal Intensive Care Unit (ICU) due to its serious condition, morbidity, and unusual evolution of the pathology in the pediatric population, scarcity of epidemiological data on the subject, still aiming address aspects of the treatment of the disease. Case report: Male patient, born on 09/11/2020 at Santa Casa de Tupã, Sao Paulo, with a gestational age of 36 weeks and 6 days, through cesarean delivery and without complications, obtaining APGAR 9 and 10, and maintained in a room with the mother. The morning after birth, the newborn (NB) began to groan and have mild respiratory distress. The patient evolved with worsening of the breathing pattern and oxygen was concentrated at 6L/min in Hood's Halo, presenting hyperthermia, hyperemia, and edema in the right elbow. Laboratory tests showed an increase in C-Reactive Protein and worsened hematimetric indices of the blood count, and antibiotic therapy was then started with oxacillin 50 mg/kg/dose and amikacin 15 mg/kg/day. The ultrasound of the right elbow showed skin thickening, absence of collections, and no changes in vasculature, being suggestive of cellulite. A new ultrasound of the right elbow showed bulging, intra-articular collection, and discreet collection with extra-articular communication in the medial region, being suggestive of septic arthritis. Blood culture collection resulted in the growth of gram-positive cocci (Staphylococcus aureus) in a pair of samples. The patient received intravenous treatment with Vancomycin for 14 days with clinical and laboratory improvement of the infection and was discharged with the use of oral Clindamycin for another 14 days and physical therapy follow-up for rehabilitation of movement of the affected limb. Final considerations: Due to the unusual evolution of SA in the neonatal period, and the scarcity of literature on the pathology in this age group, the importance of further studies on the subject, and greater surveillance for neonatal diagnosis, in addition to early treatment to reduce of complications.