Unlike secondary mitral regurgitation (MR) in the setting of left ventricular (LV) disease, the occurrence of functional MR in atrial fibrillation (AF) and/or heart failure with preserved ejection fraction (HFpEF) has remained largely unspoken. LV size and systolic function are typically normal, whereas isolated mitral annular dilation and inadequate leaflet adaptation are considered mechanistic culprits. Moreover, the role of left atrial and annular dynamics in provoking MR is often underappreciated. Because of this peculiar pathophysiology, atrial functional MR benefits from a different approach compared with secondary MR. Although both AF and HFpEF—two closely related disease epidemics of the 21st century—are held responsible, current guidelines do not emphasize the need to differentiate atrial functional MR from (ventricular) secondary MR. This review summarizes the prevalence and prognostic importance of atrial functional MR, providing mechanistic insights compared with those of secondary MR and suggesting potential therapeutic targets.