Acute pericarditis is caused by inflammation of the pericardial sac and is characterized by sharp and pleuritic chest pain, which is accompanied by a pericardial friction rub and serial electrocardiogram (ECG) changes. Although there are many known etiologies of pericarditis, we present the first known case of a herbal supplement, the Tejocote root, causing acute pericarditis in a previously healthy 23-year-old female. Her ECG showed t-wave inversions that resolved with colchicine and non-steroidal anti-inflammatory drug therapy. Prior studies have demonstrated that it can cause adverse cardiovascular effects, but this is the first documented case of the Tejocote root causing acute pericarditis. This case report reinforces the importance of inquiring about supplements patients may be taking, especially when looking for the etiology of pericarditis.