<p>Letermovir is a cytomegalovirus (CMV) prophylactic agent that is not associated with clinically significant myelosuppression. However, real-world data regarding its clinical use in Chinese allogeneic hematopoietic stem cell transplantation (allo-HSCT) remain limited. This study aimed to assess CMV DNAemia during letermovir prophylaxis and its associations with other viral infections and clinical outcomes. We conducted a single-arm retrospective study of consecutive allo-HSCT recipients treated between July 2022 and December 2023 at the First Affiliated Hospital of Xi’an Jiaotong University. Patients who received letermovir within 28 days post-transplant were analyzed. The primary endpoint was the&#xa0;100-day cumulative incidence of CMV DNAemia. Secondary endpoints included CMV end‑organ disease, acute graft‑versus‑host disease (aGVHD), other viral infections, and survival outcomes. Among 74 allo-HSCT recipients receiving letermovir prophylaxis, the 100‑day cumulative incidence of CMV DNAemia was 18.9% (95% CI, 10–28%), which remained similar after exclusion of CMV serostatus donor‑negative/recipient‑positive(D-/R+) cases (19.4%). The 100‑day cumulative incidence of aGVHD was 17.6%. In competing‑risk analyses treating death as a competing event, time‑dependent associations were observed between CMV DNAemia and aGVHD. Lower letermovir exposure (<i>p</i> = 0.036), delayed initiation of prophylaxis(<i>p</i> = 0.041), and use of high‑dose corticosteroids (<i>p</i> = 0.020) were more frequently observed among patients with breakthrough CMV DNAemia. This real‑world study describes the incidence and clinical characteristics of early CMV DNAemia in allo‑HSCT recipients receiving letermovir prophylaxis. Lower letermovir exposure and the use of high‑dose corticosteroids were more commonly observed among patients with breakthrough CMV DNAemia, highlighting clinical factors that warrant attention in this setting.</p>