According to the Valve Academic Resortium, underweight is one parameter in the definition of frailty, which is associated with increased mortality after transcatheter aortic valve implantation (TAVI) and surgical aortic valve replacement (SAVR). Aims of our study were (1) to examine the impact of underweight on mortality after TAVI and SAVR and (2) to determine the effect of intervention mode (TAVI vs SAVR) on mortality in underweight patients from the German Aortic Valve Registry. Overall, 35,109 patients treated with TAVI or SAVR were studied. Outcomes of underweight (body mass index [BMI] <20 kg/m2) TAVI and SAVR patients were compared using propensity score weighting. Prevalence of underweight was 5.7% in patients who underwent TAVI and 2.9% in patients who underwent SAVR. Underweight patients had significantly increased mortality rates for both treatment strategies compared with normal weight patients (BMI 20 to 30 kg/m2). Comparing underweight TAVI and SAVR-patients using propensity score weighting, no statistically significant differences regarding mortality rates were observed. Subgroup analysis of severely underweight patients (BMI <18.5 kg/m²) revealed no significant increase of mortality after TAVI compared with underweight patients (BMI <20 kg/m2), whereas severely underweight SAVR patients showed twofold increased mortality rates. In conclusion, underweight in patients who underwent TAVI or SAVR is rare, but it is associated with increased mortality. Especially severely underweight SAVR patients showed excess mortality rates.