Background There are conflicting results over the improvement rate and predictors of mitral regurgitation in patients undergoing transcatheter aortic valve replacement (TAVR). Purpose To define the cause, degree of improvement, and improvement predictors of moderate to severe mitral regurgitation in patients undergoing TAVR by using a simplified D-shaped mitral annulus model derived from multisection CT (MSCT). Materials and Methods This retrospective cohort study included 528 consecutive patients who underwent TAVR between April 2012 and October 2019. Patients with previous surgical aortic valve replacement and those with moderate or severe mitral stenosis were excluded. A total of 104 patients with moderate to severe mitral regurgitation met the inclusion criteria and were included in the final analysis. At least one grade reduction in the severity of mitral regurgitation was considered indicative of mitral regurgitation improvement after TAVR. Up to 5-year post-TAVR follow-up of mitral regurgitation improvement was evaluated. Mitral annular dimensions (annular area, circumference, and trigone-to-trigone, intercommissural, and anteroposterior distances) and annular calcification were assessed at MSCT with use of dedicated postprocessing software. Associations with mitral regurgitation improvement after TAVR were explored. Results A total of 104 patients with concomitant mitral regurgitation who underwent TAVR (mean age, 74 years ± 7; 61 men) were included in the study. Mitral regurgitation improved in 79 patients after TAVR and remained unchanged in the remaining 25 patients. Maximum improvement was observed in the 1st year after TAVR. D-shaped mitral annular parameters, including annular circumference (odds ratio [OR], 1.05; 95% CI: 1.01, 1.1; P = .02) and trigone-to-trigone (OR, 1.2; 95% CI: 1.03, 1.39; P = .02) and intercommissural (OR, 1.15; 95% CI: 1.02, 1.31; P = .02) distances, were related to mitral regurgitation improvement. In addition, patients with coronary artery disease had greater improvement in mitral regurgitation after TAVR (OR, 0.17; 95% CI: 0.04, 0.76; P = .02). Primary mitral regurgitation (OR, 5.1; 95% CI: 1.1, 24; P = .04) and D-shaped annular circumference (OR, 1.06; 95% CI: 1, 1.11; P = .04) were independent predictors of less mitral regurgitation improvement after TAVR. Conclusion Concomitant mitral regurgitation in patients undergoing transcatheter aortic valve replacement (TAVR) tends to improve after the procedure, with maximum improvement in the 1st year after TAVR. D-shaped annular circumference and primary mitral regurgitation were independent predictors of less mitral regurgitation improvement after TAVR. © RSNA, 2021 Online supplemental material is available for this article. See also the editorial by Collins in this issue.