We report a case of a 4-year-old female with Sotos syndrome (cerebral gigantism) whose initial clinical, pathologic, and imaging presentation was considered suspicious for a vascular malformation of her left thigh. Following 17 months of attempted treatment, excision of the supposed vascular malformation was performed. Pathology tests revealed high-grade sarcoma. The delay of diagnosis resulted in an above-the-knee amputation for definitive treatment. If this etiology had been considered earlier in this patient's clinical course, her treatment could have commenced sooner, and amputation of her leg may have been avoided. While soft-tissue sarcoma arising in childhood is rare, malignancy should be given consideration when evaluating a mass in a young child with characteristic physical examination findings of Sotos syndrome, since these children have an elevated risk of malignancy over the general population.