Abstract Objectives: To assess the effectiveness of a targeted intervention using a collaborative approach, added to a comprehensive educational intervention, to facilitate the appropriate use of oral third-generation cephalosporins (3GCs). Design: Quasi-experimental study. Setting: The University of Tokyo Hospital, a tertiary-care teaching hospital. Participants: Approximately 2,000,000 outpatients and 80,000 inpatients at the hospital between April 2017 and March 2020. Intervention: The targeted intervention using the collaborative approach was implemented in the departments with the highest use of oral 3GCs (ophthalmology and dermatology departments). Interrupted time-series analysis was applied to assess the change in days of therapy (DOT) of oral 3GCs between the preintervention period (April 2017–April 2019) and the postintervention period (May 2019–March 2020) for both inpatients and outpatients. Results: After the introduction of the targeted intervention with oral 3GCs, a significant immediate reduction of 13.48 DOT per 1,000 patient days was detected in inpatients (P < .001). However, no significant change in slope was observed before and after the intervention (−0.02 DOT per 1,000 patient days per month; P = .94). Although a temporary increase was observed after the targeted intervention in outpatients, the slope significantly decreased (−0.69 DOT per 1,000 outpatient visits per month; P = .044). No differences were observed in the use of other oral antibiotics after the intervention. Conclusions: The targeted intervention contributed to a reduction in DOT of oral 3GCs in both inpatients and outpatients. Targeted interventions using a collaborative approach might be helpful in further decreasing the inappropriate use of antibiotics.