Injection Site Sarcomas are soft tissue sarcomas affecting primarily cats, found at sites of previous injection and subsequent inflammation. These tumors are locally aggressive and treatment requires wide surgical margins. This case report will briefly review clinical presentation, diagnosis, and treatment of a 12-year old, neutered male, domestic shorthair cat who was referred to the Cornell University Hospital for Animals Oncology service for evaluation of a subcutaneous mass on his left lateral thigh which the owners had noticed 5 months prior. An incisional biopsy identified a soft tissue sarcoma. Given the species and location, a vaccine induced sarcoma was a primary differential. Computed tomography (CT) was performed with radiation treatment planning. The owners elected to surgically excise the mass with wide margins via coxo-femoral disarticulation of the left hind limb. Histopathology of the limb supported a diagnosis of an injection site sarcoma (ISS).