Investment efficiency, as one of the important criteria in assessing the quality of companies' financial decisions, has a special place in accounting and finance studies. Therefore, identifying the factors affecting this concept has always been of interest to researchers. In recent years, investment in marketing activities has been considered as one of the important forms of investment in intangible assets; however, empirical evidence regarding its effect on investment efficiency and its interaction with the capital structure of companies is still limited. Accordingly, the present study aimed to investigate the effect of investment in marketing on companies' investment efficiency and its moderating role in the relationship between financial leverage and investment efficiency. In this regard, information on 189 companies listed on the Tehran Stock Exchange was collected during the period 1393 to 1403, which includes a total of 1890 company-year observations. To test the research hypotheses, the relevant models were estimated using the fixed effects approach and the generalized least squares method. The results of the study show that investment in marketing activities has a positive and significant effect on the investment efficiency of companies. Also, the findings indicate that investment in marketing has a moderating role in the relationship between financial leverage and investment efficiency; so that increasing the level of investment in marketing activities reduces the severity of the negative effect of financial leverage on investment efficiency. In general, the results indicate that investment in marketing can provide the basis for improving the quality of investment decisions by reducing information asymmetry and strengthening the company's future performance outlook. These findings also provide new empirical evidence regarding the importance and strategic role of intangible assets, especially marketing investments, in improving the investment efficiency of companies.