Background/Objectives: Cytomegalovirus (CMV) reactivation is a major cause of morbidity and mortality in allogeneic hematopoietic stem cell transplant (allo-HCT) recipients. Letermovir is used to prevent infections, but it is not available in many countries. Ganciclovir and valganciclovir carry a risk of bone marrow suppression. Foscarnet is frequently used as a bone marrow-sparing alternative; however, its clinical utility is constrained by nephrotoxicity. In this study, we aimed to evaluate the efficacy and renal safety of foscarnet and to identify factors associated with acute kidney injury (AKI) occurring during foscarnet treatment. Methods: This real-world, retrospective, single-center cohort study included 40 adult allo-HCT recipients treated with foscarnet for CMV reactivation between May 2022 and February 2025. Results: CMV polymerase chain reaction (PCR) negativity was achieved in 90% of patients, with a median time to PCR negativity of 10 (4–28) days. AKI occurred in 42.5% of patients, and 12.5% required hemodialysis. Foscarnet treatment was associated with a significant increase in serum creatinine levels and significant reductions in potassium, calcium, and magnesium concentrations (p < 0.05 for all). In univariable logistic regression analysis, myeloablative conditioning was associated with higher odds of AKI (OR 10.29, 95% CI 1.15–91.63; p = 0.037), while male sex showed numerically higher odds that did not reach conventional statistical significance (OR 4.28, 95% CI 0.96–19.01; p = 0.056). Conclusions: Among allo-HCT recipients treated with foscarnet, virological clearance was frequently observed, while AKI and electrolyte disturbances were common. These findings suggest that close renal and electrolyte monitoring may be warranted. However, given the retrospective, single-center design, small sample size, and absence of a comparator group, the findings should be interpreted cautiously and confirmed in larger prospective comparative studies.