Functional tricuspid regurgitation (TR) is a serious pathology to be noted for severe right heart failure (HF) and poor prognosis; however, the conventional assessment of TR has some limitations and the optimal timing of surgical intervention remains unclear. A 79-year-old Japanese female was admitted to our hospital to undergo cardiac surgery, because edema gradually got worse despite the increase in diuretics. She had a history of atrial fibrillation (AF) and chronic HF due to severe TR and had been treated with a furosemide for leg edema 4 years ago. A transthoracic echocardiogram (TTE), transesophageal echocardiogram, cardiac magnetic resonance imaging, and cardiac pool scintigraphy demonstrated severe functional TR with tricuspid annular dilation, insufficient tricuspid valve coaptation, and reduced right ventricular ejection fraction (EF) but preserved left ventricular EF. In addition, Swan-Ganz catheter study showed normal pulmonary arterial wedge pressure and mean pulmonary arterial pressure. Tricuspid ring annuloplasty was performed with MC3 ring. Postoperative TTE showed trivial TR, and she had no edema with normal sinus rhythm two months later. Annuloplasty to severe functional TR caused by tricuspid annular dilation due to AF dramatically improved right HF. Cardiologist should pay strict attention to the optimal timing of surgical intervention for TR.