The bridge crossing the Gesso River in Cuneo (Piedmont, Italy) is a multi-span masonry arch bridge built in the 19th century. Due to various damage phenomena, the bridge recently underwent significant strengthening interventions. Ambient vibration tests were performed before and after the repairs to assess the intervention's effectiveness and effects. A state-of-the-art output-only identification technique was adopted to evaluate the dynamic characteristics of the bridge. The pre-intervention analysis clearly revealed the presence of structural anomalies, including a sort of “frequency splitting” as well as irregularities in the mode shapes localized in areas with decayed masonry. After the strengthening works, the identified modal parameters showed a marked increase in natural frequencies, along with the resolution of previously identified mode shape irregularities, conceivably indicating the improvement in the structural condition of the bridge. The paper highlights the value of operational modal analysis as a non-destructive tool for assessing the structural condition of historical infrastructures as well as for validating the effectiveness of rehabilitation measures.