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Respiratory syncytial virus (RSV) is the leading cause of bronchiolitis in children < 24 months and a major public health concern, causing high rates of Emergency Department (ED) visits, hospitalizations, and Pediatric Intensive Care Unit (PICU) admissions. Nirsevimab is a recombinant monoclonal antibody recommended for all infants and high-risk children < 24 months. A retrospective single-center cohort study was conducted to evaluate the impact of nirsevimab introduction on bronchiolitis epidemiology in an Italian tertiary pediatric ED, accounting for 40,000 admissions/year. All children < 24 months who presented to our ED with bronchiolitis during two consecutive RSV seasons (first season: 1 October 2023 to 30 April 2024; second season: 1 October 2024 to 30 April 2025) were included. Descriptive and multivariate analyses are reported. Overall, 484 patients were analyzed (336 in 2023-2024; 148 in 2024-2025), with immunization coverage reaching 87.5% by April 2025. Compared with the previous season, RSV positivity decreased significantly (32.4% vs. 47.9%; p = 0.003) and was lower in immunized children (16.2% vs. 51.5%; p < 0.001). Immunization was associated with a reduced risk of RSV-positive swab in the second season (OR = 0.159, 95% CI: 0.059-0.397). Among RSV-negative patients, other respiratory viruses increased (p < 0.001), while co-infections increased in RSV-positive cases (p = 0.021). Hospitalization rates remained stable, though absolute admissions were halved. In conclusion, nirsevimab immunization reduced RSV burden, supporting its inclusion in universal prevention programs and the need for multicenter prospective studies to assess long-term outcomes.