Purpose: The study aims to extend the conventional triple bottom line (TBL) framework (prosperity, people and planet) to the quadruple bottom line (QBL) by newly adding a “prophet” dimension for Islamic banks seeking compliance with Islamic law in their pursuit of sustainability. Design/methodology/approach: Employ Chapra's corollaries of maqasid al-shari'ah (the goals of Islamic law) to develop constructs for a survey of 504 Islamic bank stakeholders from five Indonesian provinces to gather primary data to quantitatively verify the dimensions and items in the proposed QBL framework. Categorical principal component analysis (CATPCA) then identifies the sustainability of ten Islamic banks from ten countries as a trial application of the resulting QBL index. Findings: Using the dimensions and items identified using CATPCA, the authors develop a QBL index to assess the sustainability of the ten Islamic banks. The findings suggest that half of the banks are sufficiently sustainable, with three being proactive (doing more than is required) and two being accommodative (doing all that is required). The remaining five banks are unsustainable, with two banks being defensive (doing the least that is required) and three being reactive (doing less than is required). Most of the banks perform relatively poorly according to the “planet” (38%) and “people” (41%) dimensions and perform better on the “prosperity” (53%) and “prophet” (63%) dimensions. Nonetheless, there is ample room for improvement across all dimensions of sustainability. Research limitations/implications: The generalizability of the findings is limited by the small-scale single-country survey used in the CATPCA part of the analysis. Only ten Islamic banks were included in the QBL scoring and ranking exercises Practical implications: Islamic banks can improve their sustainability by increasing green financing and reaching out to rural areas and disadvantaged populations. In countries with Islamic banking systems, regulators can support this through training, guidance and incentives. Originality/value: Pioneering exploration of TBL from maqasid al-shari'ah perspective. First, we develop a QBL index to assess the sustainability of Islamic banks in line with actual stakeholder expectations.