Septic arthritis is the infection of a joint by an infectious agent, leading to arthritis. It is therefore important to identify and treat the correct bacteria in septic arthritis. However, accurate identification of bacteria by conventional methods is difficult because of the distinct biochemical characteristics of individual bacteria. This case report aims at assessing septic arthritis caused by Streptococcus dysgalactiaesubsp. equisimilis(SDSE) using nucleotide sequences and discusses the associated treatment. Here, Streptococcus agalactiaewas determined to be the causative bacteria for septic arthritis in a 77 year-old woman using the conventional method of hemolysis pattern interpretation and morphology. However, nucleotide sequence analysis of 16S ribosomal RNA revealed that SDSE was the causative strain. 16S rRNA gene sequencing can correctly identify bacteria strains that are difficult to be identified by traditional method, and this correct identification can provide patients with the opportunity for adequate treatment using the proper antibiotics.