Background Several clinical trials have shown that nirsevimab, an antibody targeting the respiratory syncytial virus (RSV), reduces RSV-bronchiolitis requiring admission. In 2023-2024, Catalonia and Andorra adopted immunization strategies for children < 6 months and those born during the epidemic season. This study evaluates the effectiveness of nirsevimab in preventing hospitalizations from RSV bronchiolitis. Methods In the epidemic season of 2023-2024, a test-negative case-control study was conducted in three hospitals from Catalonia and Andorra. Patients <12 months old admitted with bronchiolitis and tested for RSV using molecular microbiology tests were included. The effectiveness in preventing RSV-bronchiolitis hospitalization and severe disease was estimated using multivariate models. Comparisons between immunized, non-immunized and non-eligible patients were made in prospectively collected epidemiological, clinical, and microbiological variables. Results 234 patients were included. RSV was detected in 141/234(60.2%), being less common in the immunized group (37% vs 75%, p<0.001). The rate of immunized patients among those eligible was 59.7%. The estimated effectiveness for RSV-associated lower respiratory tract infection was 81.0% (95% confidence interval: 60.9-90.7), and for preventing severe disease (the need for NIV/CMV), 85.6% (41.7-96.4%). No significant differences by immunization status were observed in patients with RSV concerning viral co-infections, the need for NIV/CMV or length of hospital stay. Conclusions This study provides real-world evidence of the effectiveness of nirsevimab in preventing RSV-LRTI hospitalization and severe disease in infants during their first RSV season following a systematic immunization program. Immunized patients did not exhibit a higher rate of viral co-infections nor differences in clinical severity once admitted.