How does managerial opportunism affect corporate investment efficiency? Prior research establishes corporate investment efficiency as a function of the firm’s information environment and internal governance. We examine how managerial opportunism is an agency conflict that distorts corporate investment policy. We use an ex-ante firm level measure of managerial opportunism proxied with the insider trading activity of top executives and test its effects on firm investment efficiency and performance. Our results show that managerial opportunism decreases firm investment efficiency and has negative effects on both accounting and stock performance. Further tests show that both the quality of the information environment and internal governance moderate the negative effects of managerial opportunism, providing a unique perspective on how insider trading policy and regulation can affect corporate investment policy. Our results are robust to alternative proxies for firm governance, which includes board co-option, alternative model specifications, and tests for endogeneity.