Coronavirus disease 2019 (COVID-19) is a disease characterized by predominantly respiratory symptoms, which can progress to respiratory failure. Due to the novelty of the vaccines, it is difficult to assess if there are any associated long-term side effects. Here, we present a case of an elderly female who received the Moderna COVID-19 vaccine and developed a high-grade sarcoma at the site of the injection. A 73-year-old female with a past medical history of hypertension, hyperlipidemia, and renal angiomyolipoma status post resection in 2019 presented with worsening right upper arm swelling for the past two weeks. She noticed the swelling two to four days after receiving her second dose of the Moderna vaccine within 1 cm from the prior injection site. Physical examination was remarkable for a 6 cm, circular, mobile, soft mass present in the right upper arm. MRI with and without contrast revealed a 5.2 cm soft tissue mass overlying the triceps region with irregular features concerning for malignancy. Fine needle aspiration revealed pathologic characteristics indicative of high-grade sarcoma. The patient ultimately had resection of the mass four months after the initial visit and was diagnosed as having grade 3, stage IIIA undifferentiated, pleomorphic high-grade sarcoma. Herein, we present a case demonstrating the development of high-grade sarcoma at the injection site in an elderly female patient within days of receiving the second dose of the Moderna COVID-19 vaccine. Currently, it is unclear whether there is a true association between the vaccines and malignancy or inflammatory response exacerbating underlying malignancy. This case highlights the necessity to investigate and be aware of such rare, adverse complications that may be associated with the novel COVID-19 vaccinations to guide physicians in their differential diagnosis.