An eight-year-old Standardbred gelding presented to the Cornell University Equine Hospital for a one-week history of poor performance due to presumptive atrial fibrillation. On presentation he had an irregularly irregular heart rhythm and tachycardia. Atrial fibrillation was diagnosed on electrocardiogram. His plasma cardiac troponin I level was normal and an echocardiogram showed no significant structural abnormalities. He underwent medical cardioversion with quinidine sulfate, and converted to normal sinus rhythm after three doses. He was monitored with a telemetric electrocardiogram system throughout the procedure, and an electrocardiogram was performed the day after conversion to confirm that he had stayed in normal sinus rhythm. This case report will discuss the clinical presentation, diagnostic tests, treatment options, and prognosis associated with atrial fibrillation in racehorses as well as risk factors for developing the disease and recurrence following conversion.