Background: Cardiovascular complications are increasingly recognized in patients undergoing hematopoietic stem cell transplantation (HSCT). Early detection of subclinical myocardial dysfunction may improve risk stratification, and global longitudinal strain (GLS) is emerging as a sensitive marker of early cardiac impairment. Methods: We conducted a single-center observational cohort study including 518 adult patients undergoing autologous (n = 64) or allogeneic (n = 454) HSCT between 2004 and 2025. Baseline cardiovascular risk factors, transplant characteristics, and echocardiographic parameters—including GLS in a subset—were recorded. Abnormal GLS was defined as less negative than −20%. The primary outcome was the occurrence of cardiovascular events (composite of cardiovascular death, myocardial infarction, stroke, atrial fibrillation/flutter, pericardial effusion, pulmonary embolism, and left ventricular systolic dysfunction). Multivariable logistic regression was used to identify independent predictors. Results: Median age was 53 years; 58% were male. Cardiovascular events were predominantly atrial fibrillation, pericardial effusion, and reduced left ventricular function, whereas ischemic events were rare. Over 90% of events occurred within 100 days post-transplant. Multivariable analysis identified older age (OR 1.28 per 10-year increment; 95% CI 1.10–1.48; p = 0.002), chronic kidney disease (OR 2.44; 95% CI 1.18–5.02; p = 0.01), pre-transplant atrial fibrillation (OR 2.12; 95% CI 1.04–4.31; p = 0.03), and abnormal baseline GLS (OR 1.89; 95% CI 1.02–3.52; p = 0.04) as independent predictors. Importantly, the prognostic value of GLS remained significant after excluding clinically insignificant pericardial effusions from the composite endpoint. GLS deterioration during follow-up occurred more frequently in patients receiving reduced-intensity conditioning compared with myeloablative conditioning (25% vs. 12.7%; p = 0.006). Conclusions: Subclinical myocardial dysfunction detected by GLS identifies HSCT recipients at increased cardiovascular risk. These findings support the incorporation of strain imaging into routine pre- and post-transplant cardiovascular evaluation to enable earlier detection and guide targeted interventions.