Pediatric osteomyelitis commonly occurs in the long bones and has rarely been reported in small bones such as the vertebrae and ribs. Rib osteomyelitis occurs in approximately 1% or less of all cases of hematogenous osteomyelitis, and is usually caused by Staphylococcus aureus. We present a case of acute osteomyelitis in the rib of an otherwise healthy and afebrile 1-year-old infant that appeared as a lytic bone lesion on imaging studies and was mistaken for a tumor. Biopsy was needed to rule out malignancy and establish the diagnosis of osteomyelitis caused by group A Streptococcus. This is one of the few reported cases of pediatric rib osteomyelitis caused by this organism.