This study examines the effect of the CEO structure on R&D investment efficiency. The co-CEO structure distributes decision-making authority among several CEOs. Shared leadership of the structure can reduce an individual CEO’s hegemonic power and prevent agency problems. Therefore, the co-CEO structure can act as a different governance mechanism. Using a sample of publicly traded companies from 2013 to 2019, we find that firms appointing multiple CEOs engage in less over-investment. Next, firms with a co-CEO structure engage in less over-investment when they have at least one owner-manager. Our results contribute to the literature by providing evidence that the co-CEO structure can serve as a means of mutual monitoring.