This article investigates the relationship between the share of assets held by institutional investors relative to GDP and a synthetic index of union bargaining power in 20 OECD countries from 1980 to 2017. Findings from the use of fixed-effects OLS and instrumental variables regressions show that the share of assets held by institutional investors, especially insurance companies and pension funds, is correlated with a decline in union bargaining power. The authors argue that by contributing to significant changes in corporate strategies and governance through potential effects on jobs and wages, institutional investors in most OECD countries may have weakened the influence of trade unions, thereby leading to a higher decentralization of wage bargaining. Mixed evidence is found, however, when investigating the role of complementarity across institutions to explain cross-country heterogeneity.