Citrobacter infections are reported in neonates and immunocompromised patients most commonly. Citrobacter koseri (diversus) is an important cause of neonatal meningitis and brain abscess formation. But osteomyelitis is a rare condition and very few cases were reported. We described a 68-years-old man with diabetes mellitus who had 1st metatarsal osteomyelitis due to Citrobacter koseri. Intravenous treatment with piperacillin-tazobactame was administered for 3 weeks and the wound was recovered completely. Citrobacter koseri can be a cause of osteomyelitis in patients with underlying predisposing factors and bone cultures should be evaluated to detect probable pathogens and determine proper antimicrobial therapy.