Antimicrobial resistance represents a major global health challenge, and hospital-based antimicrobial stewardship programs (ASPs) are essential for promoting appropriate antibiotic use. This study aimed to introduce and implement an ASP in a university hospital and to assess its impact on antibiotic prescribing patterns and patient outcomes. We conducted a single-center study including infectious disease (ID) consultations performed before ASP introduction (2023), during implementation (2024), and after full implementation (December 2024–February 2025). Primary outcomes were carbapenem prescriptions and antimicrobial stewardship interventions, while secondary outcomes included in-hospital mortality, 30-day in-hospital mortality, and length of hospital stay (LOS). A total of 1673 ID consultations involving 975 patients in pre- and post-interventional phases were analyzed. Carbapenem prescriptions progressively and significantly declined over the study phases (from 32.1% to 18.6%; adjusted OR 0.57 [95% CI 0.35–0.92] in Phase II and 0.39 [95% CI 0.22–0.70] in Phase III compared to Phase I). Antimicrobial stewardship interventions increased significantly over time (reaching 54.0% in Phase III; adjusted OR 1.94, 95% CI 1.40–2.69). No significant differences in 30-day in-hospital mortality or LOS were detected across the study phases. In conclusion, a structured ASP was associated with improved antibiotic prescribing practices, supporting its role as a key strategy for optimizing antimicrobial use in hospital settings.