The primary objective of this study is to examine the impact of sustainability disclosure on firm value and investor behavior among companies listed in the capital market. To this end, using data from a sample of firms over a specified period and employing a multiple regression model, the relationship between sustainability disclosure and the dependent variables was empirically tested. sustainability disclosure was designated as the independent variable, while firm value and investor behavior served as the dependent variables. The results of the hypothesis tests indicate that the regression model exhibits an adequate goodness of fit, and the independent variables explain a substantial proportion of the variance in the dependent variable. Furthermore, the Durbin–Watson test results confirm the absence of autocorrelation, and the Variance Inflation Factor values indicate no multicollinearity among the explanatory variables. The findings reveal that sustainability disclosure exerts a statistically significant effect on firm value and investor behavior; however, the direction of this effect is negative. This outcome suggests that, within the studied sample, investors perceive sustainability disclosures as cost-incurring or as factors that diminish short-term returns, leading to a negative market reaction. Conversely, variables such as ROA, firm size, firm age, and financial leverage demonstrate a positive and significant impact on the dependent variable, whereas firm growth and industry exert no significant effect. Overall, the results indicate that while sustainability disclosure statistically influences firm value and investor behavior, investor perception and interpretation of such information play a decisive role in shaping the direction of this effect.