Objective: Invasive Streptococcus agalactiae infection in nonpregnant women has been reported increasingly worldwide. This study reports the clinical features and outcome of S. agalactiae septic arthritis in Thai patients. Methods: The medical records of cases with septic arthritis seen between July 1990 and December 2010 were reviewed. Only those with S. agalactiae were included in this study. Results: From 244 cases of septic arthritis, 38 (15.57%, 13 men and 25 women) were caused by S. agalactiae, with 34 of them (89.48%) occurring between 2008 and 2010. Their mean age was 52.89 (SD, 18.95) years. Twenty-four of the 38 patients (63.16%) had 1 or more underlying disease that might predispose to joint infection. Fever and joint pain were the most common symptoms presented. Eleven cases (28.95%) presented monoarthritis, 15 (39.47%) oligoarthritis, and 12 (31.58%) polyarthritis, with a mean joint involvement of 3.34 (SD, 2.35) joints (range, 1-8). Cellulitis was seen in 27 cases (71.05%). Blood cultures were positive in 31 patients (81.58%). Thirty-five of the 38 synovial fluid specimens obtained were enough for cultures and stain smears, with 24 (68.57%) growing S. agalactiae and 19 (54.29%) showing gram-positive cocci. All isolates were sensitive to penicillin. Ten patients (26.31%) received arthroscopic drainage. The articular outcome was good in 11 patients, fair in 24, and poor in 3. There were no deaths. Conclusions: Streptococcus agalactiae is an emerging cause of septic arthritis in Thai patients. Physicians should be especially aware of this condition in patients presenting with acute oligopolyarthritis and prominent cellulitis. Copyright © 2014 by Lippincott Williams & Wilkins.