Investment efficiency is considered one of the key indicators of the quality of firms’ financial decision making, and identifying its determinants has been a central topic in accounting and finance research. In recent years, investment in marketing activities has increasingly been recognized as an important form of intangible assets. However, empirical evidence regarding its role in improving investment efficiency, as well as its interaction with firms’ capital structure, remains limited. This study aims to examine the effect of marketing investment on firms’ investment efficiency and to investigate its moderating role in the relationship between financial leverage and investment efficiency. To this end, data from 189 firms listed on the Tehran Stock Exchange over the period 2014–2024 were collected, resulting in a total of 1,890 firm–year observations. The research hypotheses were tested using a multivariate regression model within a panel data framework, and the models were estimated using the fixed effects approach and the generalized least squares (GLS) method. The findings indicate that investment in marketing has a positive and statistically significant effect on firms’ investment efficiency. Moreover, the results reveal that marketing investment plays a moderating role in the relationship between financial leverage and investment efficiency, such that higher levels of investment in marketing activities mitigate the negative impact of financial leverage on investment efficiency. Overall, the results suggest that marketing investment can enhance the quality of investment decisions by reducing information asymmetry and improving firms’ future performance prospects. These findings provide new empirical evidence on the strategic role of intangible assets in improving corporate investment efficiency.