Solitary fibrous tumor (SFT) is uncommon and known to affect in middle-aged adults, with rare reports of occurrences in children. Although frequently involving the pleura, recent reports show that SFT may also involve extrapleural sites. SFT is known as a benign tumor, but in some cases SFT recurs, invades locally, or shows malignant transformation. We experienced a case of SFT that developed in the axilla of a 4-year-old girl with multiple congenital anomalies with constitutional chromosomal abnormality of 46,XX[44]/47,XX,+mar[11]. Her tumor could not be surgically resected due to the patient’s multiple anomalies including congenital heart disease, chronic lung problem and seizure disorder. She died of congestive heart failure with respiratory failure due to tumor growth. This report is meaningful not only because SFT affected a pediatric patient with a constitutional chromosomal abnormality, but also because the tumor originated from the axillary area, a site of origin rarely reported for SFTs.