Background: Atrial functional tricuspid regurgitation (atrial TR) has received growing recognition as a TR entity with a distinct cause owing to its independence from valvular tethering as the predominant mechanism underlying TR. However, characterization of atrial TR varies, and a universal definition is lacking. Methods: In total, 651 patients with significant functional TR were analyzed, including 438 conservatively treated individuals and 213 patients who received transcatheter tricuspid valve repair (TTVR). Based on a clustering approach, we defined atrial TR as tricuspid valve (TV) tenting height ≤10 mm, midventricular right ventricular diameter ≤38 mm, and left ventricular ejection fraction ≥50%. Results: Patients with atrial TR were more often females, had higher right ventricular fractional area change, higher left ventricular ejection fraction, and lower LV end-diastolic diameter, TV tenting area and height, lower right ventricular and tricuspid annular size, enlarged, but lower right atrial area and lower TV effective regurgitant orifice area (all P <0.05). Patients with atrial TR had significantly better long-term survival than non-atrial TR in the conservatively treated TR cohort ( P <0.01, n=438). Atrial TR was independently associated with a lower rate of the combined end point of mortality and heart failure hospitalization at 1-year follow-up in the TTVR cohort (hazard ratio, 0.39; P <0.05, n=213). TR degree was significantly reduced after TTVR in non-atrial and atrial TR ( P <0.01). Functional parameters significantly improved following TTVR independent of TR cause ( P <0.05). Conclusions: An echocardiography-based atrial TR definition is associated with prognostic relevance in patients with functional TR in conservatively treated TR and after TTVR.