We report a case of septic arthritis in the shoulder of an infant treated with a combination of arthroscopic irrigation, debridement, and synovectomy. The results were encouraging. The patient was a boy who was 2 years 6 months of age. His first symptoms were a body temperature of 40 degrees C and right upper arm pain. Five days after the onset, he was brought to our institution. His shoulder was swollen and erythematous. Yellowish fluid (7 ml) was aspirated from his shoulder joint; the causative organism was group A Streptococcus. On admission, arthroscopic surgery was performed. His temperature gradually decreased, and inflammatory markers including the white blood cell count and C-reactive protein level improved. At his 1-year follow-up there was no sign of infection, and a full range of motion in the shoulder was maintained. The interval between the onset and the start of treatment is an important factor affecting the prognosis. The patient was brought to our institution 5 days after onset and underwent arthroscopic surgery on the same day. This early treatment was one of the factors that contributed to a successful result. Arthroscopic surgery, including irrigation and debridement, may be one of the options for treating septic arthritis of an infant's shoulder.