As jurisdictions around the world diverge in their adoption of sustainability reporting regimes and materiality approaches, it remains unclear how these institutional features jointly shape disclosure transparency. We experimentally examine how reporting regimes (voluntary vs. mandatory) and materiality approaches (financial materiality vs. double materiality) interact to influence corporate managers’ transparency in disclosing negative but material sustainability information. We find that mandatory reporting increases transparency only under double materiality, but not under financial materiality. Furthermore, under voluntary reporting, financial materiality leads to more transparent disclosure than double materiality, an effect that attenuates under mandatory reporting. These effects are driven by differences in accountability structures across materiality approaches and reporting regimes, which together shape whether managers engage in symbolic versus substantive accountability actions that in turn affect disclosure transparency. Our findings have implications for regulators seeking to enhance the effectiveness of sustainability reporting requirements.