Obesity is a major risk factor for cardiovascular disease; however, a paradoxical effect of obesity has been reported in patients with heart failure or myocardial infarction. Although several studies have suggested the same obesity paradox in patients undergoing transcatheter aortic valve replacement (TAVR), they included a limited number of underweight patients. This study aimed to clarify the effect of being underweight on TAVR outcomes. We retrospectively analyzed 1,693 consecutive patients undergoing TAVR between 2010 and 2020. The patients were categorized according to body mass index: underweight (<18.5kg/m2; n=242), normal weight (18.5 to 25kg/m2; n=1,055), and overweight (>25kg/m2; n=396). We compared midterm outcomes after TAVR among the 3 groups; all clinical events were in accordance with the Valve Academic Research Consortium-2 criteria. Underweight patients were more likely to be women and have severe heart failure symptoms, peripheral artery disease, anemia, hypoalbuminemia, and pulmonary dysfunction. They also had lower ejection fractions, smaller aortic valve areas, and higher surgical risk scores. Device failure, life-threatening bleeding, major vascular complications, and 30-day mortality occurred more frequently in underweight patients. The midterm survival rate of the underweight group was inferior to those of the other 2 groups (P< 0.0001; average follow-up, 717days). In the multivariate analysis, underweight was associated with noncardiovascular mortality (HR: 1.78; 95%CI: 1.16-2.75) but not cardiovascular mortality (HR: 1.28; 95%CI: 0.58-1.88) after TAVR. Underweight patients had a worse midterm prognosis, demonstrating the obesity paradox in this TAVR population. (Outcomes of transcatheter aortic valve implantation in Japanese patients with aortic stenosis: multi-center registry; UMIN000031133).