Cytomegalovirus (CMV) is serious viral infection in allogeneic hematopoietic cell transplantation (allo-HCT) recipients. November 2017, the novel CMV DNA terminase complex inhibitor letermovir was approved for prophylaxis of CMV infection in CMV-seropositive allo-HCT recipients. Here we sought to determine the effectiveness of letermovir in preventing CMV infection in CMV-seropositive patients undergoing haploidentical or mismatched adult unrelated donor allo-HCT using post-transplantation cyclophosphamide-based graft-versus host-disease prophylaxis. Sixty-four patients underwent transplantation between 2014 and 2019, of whom 32 received letermovir and 32 did not receive letermovir. The day 180 cumulative incidence of CMV infection requiring therapy was 45.3% (95% confidence interval CI, 32.7% to 57.1%) in the entire cohort, 68.8% (95% CI, 48.9% to 82.2%) in the patients who did not receive letermovir, and 21.9% (95% CI, 9.5% to 37.6%; P < .001) in patients who received letermovir. Adjusting for regimen intensity, disease histology, and age, the hazard ratio for CMV infection was .19 (95% CI, .08 to .47; P < .001) in patients who received primary prophylaxis with letermovir. The 1-year cumulative incidence of treatment- related mortality was similar between patients with and without letermovir treatment (16.9% versus 18.9%), as was overall survival (64.0% versus 49.0%). Persistent CMV infection requiring >28 days of therapy was more common in patients who did not receive letermovir (31.2% versus 6.2%; P = .02). In summary, letermovir was effective in preventing CMV infection in this high-risk population of HLA-mismatched allo-HCT recipients.