This study examines the relationship between institutional ownership, firm size, and firm value, with tax avoidance serving as a moderating variable. Institutional ownership and firm size are often viewed as critical factors that can influence a firm’s financial performance and its market valuation. Tax avoidance, a strategy employed by firms to reduce their tax liabilities, may alter the effect of these factors on firm value. Using data from publicly traded firms over a five-year period (2017–2022), the study employs regression analysis to test the moderating role of tax avoidance. The results indicate that both institutional ownership and firm size positively impact firm value. Moreover, tax avoidance significantly moderates these relationships, enhancing the positive effect of institutional ownership on firm value and diminishing the negative effect of firm size on firm value. These findings offer implications for investors, policymakers, and corporate managers seeking to understand the dynamics between ownership structures, firm size, and corporate strategies like tax avoidance.