The increasing number of Initial Public Offerings (IPOs) in Indonesia following the COVID-19 pandemic has been accompanied by price inefficiencies and high short-term volatility, indicating that IPO pricing does not fully reflect firms’ intrinsic value. This study aims to examine the effect of free float on short-run stock returns and to analyze the moderating role of underwriter reputation in the Indonesian capital market. Employing a quantitative explanatory approach, this research uses secondary data from companies conducting IPOs on the Indonesia Stock Exchange (IDX) during the 2021–2024 period, selected through purposive sampling. The analysis is conducted using linear regression and Moderated Regression Analysis (MRA). The findings reveal that free float has a positive and significant effect on short-run stock returns, suggesting that a higher proportion of publicly tradable shares enhances liquidity and investor demand. Furthermore, underwriter reputation significantly strengthens this relationship, indicating its role in reducing information asymmetry and improving price efficiency in the early post-IPO period. These results imply that both ownership structure and intermediary reputation are critical in shaping short-term market performance, providing valuable insights for investors, issuers, and regulators in improving IPO pricing efficiency and market stability.