Viral infections emerge in the pathogenesis of subacute thyroiditis. Aside from this, subacute thyroiditis following vaccines utilizing inactivated viruses has been shown on rare occasions. Due to the COVID-19 pandemic, several vaccines have been developed all over the world; mass and unprecedented vaccination has thus been initiated. However, it is known that cases such as subacute thyroiditis have been reported, albeit rarely, after administration of COVID-19 vaccines. In this case report, we present a 59-year-old patient with multiple myeloma developing subacute thyroiditis following BNT162b2 vaccine. Patient had swelling in the neck, and his symptoms were controlled with non-steroidal anti-inflammatory drugs. Subacute thyroiditis following administration of the COVID-19 vaccine is rare; however, it is likely an under-reported condition that is difficult to detect. Clinicians should stay informed and have increased awareness of post-COVID-19 vaccine subacute thyroiditis.