Municipal solid waste incineration (MSWI) fly ash poses environmental risks and demands sustainable management strategies. Reusing this byproduct in building materials offers a promising pathway for waste reduction and resource recovery. However, selecting the most sustainable fly ash (FA)-to-building material (BM) technology requires balancing diverse environmental, economic, and technical criteria. This task is further complicated by data gaps and inherent uncertainties, which often exclude emerging technologies, introduce bias through subjective weighting, and limit the reliability of sustainability assessments. To address these gaps, this study develops a novel decision support system (DSS) that imputes missing data using expert judgments and partial datasets, derives balanced weights by integrating subjective preferences with objective data features, and employs the PROMETHEE II outranking method to rank alternatives amid uncertainty and incompleteness. The proposed DSS enhances existing assessment methods by effectively managing information gaps and supporting robust sustainability evaluations. A case study involving seven FA-to-BM alternatives and twelve criteria demonstrates the practical applicability and decision-making capacity of the framework. The results highlight the system’s potential to assist complex decision-making processes and provide methodological insights for the sustainable reuse of MSWI fly ash in building materials.