Managers’ overconfidence leads to overestimating their ability to manage cash sources. Holding more cash may result in overinvestment in projects and investment inefficiency consequently. The present study aims to investigate the effect of cash holding on investment efficiency with the moderating role of managerial overconfidence in Iranian companies. All listed firms in Tehran Stock Exchange, excluding banks, insurance, pension funds, and financial intermediaries, are included in the research. We have used data from financial statements of 91 companies over the period from 2010 to 2018 and conducted multiple regression models to test the hypotheses based on pooled and panel data set with fixed effects. The results indicate a positive relationship between managerial overconfidence and cash holding. The effect of cash holding on investment efficiency turns out to be significantly negative. Furthermore, managerial overconfidence has a significant moderating effect on the relation of the variables. This study is almost the first one, which has been done in emerging markets, so the study’s findings not only contribute to the existing literature on managerial overconfidence and investment efficiency but also assist policymakers, managers, and investors in making effective decisions.