Importance: Data on the burden of new-onset atrial fibrillation after transcatheter aortic valve implantation (TAVI) and surgical aortic valve replacement (AVR) is limited mostly to small series or post hoc analyses of clinical trials. Objectives: To evaluate the incidence of new-onset atrial fibrillation and assess the incidence of in-hospital mortality associated with new-onset atrial fibrillation after TAVI and AVR. Design, Setting, and Participants: In this population-based observational study using the National Inpatient Sample and a validation cohort from the New York state inpatient database, the National Inpatient Sample was queried from January 1, 2012, to September 30, 2015, and the New York state inpatient database was queried from January 1, 2012, to December 31, 2014. Hospitalizations of adults undergoing TAVI or isolated AVR were examined. The incidence of in-hospital mortality across groups with new-onset atrial fibrillation was assessed in the National Inpatient Sample cohort using multivariable logistic regression modeling. Statistical analysis was conducted from August 20, 2018, to March 19, 2019. Main Outcomes and Measures: The primary outcome was the occurrence of new-onset atrial fibrillation, which was identified by excluding hospitalizations in which atrial fibrillation was present on admission. The secondary outcome was in-hospital mortality in TAVI and AVR hospitalizations with and without new-onset atrial fibrillation. Results: A total of 48715 TAVI hospitalizations (47.4% women and 52.6% men; mean [SD] age, 81.3 [8.1] years; 82.3% white) and 122765 AVR hospitalizations (39.0% women and 61.0% men; mean [SD] age, 67.8 [12.0] years; 78.0% white) were identified. New-onset atrial fibrillation occurred in 50.4% of TAVI hospitalizations and 50.1% of AVR hospitalizations. In the multivariable-adjusted model, TAVI and AVR hospitalizations with new-onset atrial fibrillation had higher odds of in-hospital mortality compared with hospitalizations without new-onset atrial fibrillation (TAVI: odds ratio, 1.57; 95% CI, 1.21-2.04; and AVR: odds ratio, 1.36; 95% CI, 1.08-1.70). The results were then confirmed with the New York state inpatient database, which contains a present on arrival indicator. The incidence of new-onset atrial fibrillation was 14.1% (244 of 1736 hospitalizations) after TAVI and 30.6% (1573 of 5141 hospitalizations) after AVR in the New York state inpatient database. Conclusions and Relevance: In this large nationwide study, a substantial burden of new-onset atrial fibrillation was observed after TAVI and AVR. The incidence of new-onset atrial fibrillation was higher after AVR than after TAVI in a patient-level state inpatient database.