Septic arthritis and osteomyelitis due to Serratia marcescens in immunocompetent patients without risk factors are extremely rare. Here, we report a case of septic arthritis and severe adjacent osteomyelitis of the tibia due to Serratia marcescens in a diabetic community-dweller patient. The patient had no contact with healthcare workers or facilities and had no chronic disease except for poorly controlled diabetes. Without predisposing risk factors, this type of infection is extremely rare, even in diabetics.