The protection of Critical Information Infrastructure (CII) faces increasingly complex challenges, necessitating collaborative approaches beyond traditional security frameworks. This study introduces an innovative Decision Support System (DSS) for CII protection, integrating cross-sectoral collaboration to produce dynamic security recommendations based on connectivity and resource similarity. Using the Knowledge Management Systems Development Life Cycle (KMSDLC) model, the DSS was developed through structured interviews with stakeholders managing national digital services. The prototype’s usability, assessed with the System Usability Scale (SUS), achieved a score of 78.75 (“Good”), indicating high acceptance. The key novelty of this study is the integration of cross-sector collaboration into DSS to enable real-time, adaptive security recommendations, filling a gap in current static DSS models. Additionally, the research contributes to cybersecurity theory by advancing DSS design with similarity-based recommendations and offers practical guidance for multi-stakeholder CIIP management. Future research should explore integrating AI for predictive analytics to enhance threat mitigation capabilities.