We present the case of a 46-year-old woman with no significant past medical history who developed left mid-thigh pain and fullness. Imaging demonstrated a mineralized soft-tissue mass, which increased in size during a year of monitoring, but retained a circumscribed appearance. The mass was located in the medial soft tissues of the thigh, separate from the bone on imaging studies, and this finding was confirmed during excision. The mass showed gross and microscopic features of an aneurysmal bone cyst. This diagnosis was supported by cytogenetic analysis revealing a t(17;17)(p13;q21) translocation corresponding to the USP6 and COL1A1 loci. Soft-tissue aneurysmal bone cyst is a rare entity, with fewer than 25 reports in the literature. Limited cytogenetic information about these tumors is available. To our knowledge, the USP6 and COL1A1 rearrangement has only previously been described in a pediatric soft-tissue aneurysmal bone cyst. We also discuss the differential diagnosis of ossifying soft-tissue lesions.