We report herein on a case of community-acquired necrotizing soft tissue infection caused by Serratia marcescens. The patient had been treated with prednisolone, tocilizumab and tacrolimus for rheumatoid arthritis. Since Gram staining of the tissue revealed Gram negative rod bacteria, ceftriaxone and clindamycin were administered as empiric therapy. Tissue culture revealed S. marcescens. Ceftriaxone was continued according to the antibiotic sensitivity. She underwent debridement of necrotic tissue and continued ceftriaxone for 17 days. She recovered and was discharged after skin grafting.