Green roofs (GRs) serve as resilient and adaptive nature-based solutions, significantly enhancing ecosystem services when retrofitted onto existing buildings in high-density urban environments. Numerous studies have proposed various methodologies and criteria to assess the green roof retrofitting potential (GRRP). However, a systematic review of assessment methodologies, indicators, and associated data remains lacking, hindering the advancement of evidence-based spatial planning. This study systematically reviews 132 peer-reviewed publications on GRRP to identify prevailing trends, synthesize methodological approaches, highlight existing knowledge gaps, and propose directions for future research. The findings reveal a relatively limited focus on GRRP assessment in tropical and desert climate zones, developing countries, and regions in the southern hemisphere. Future research should prioritize retrofitting building roofs with multifunctional systems to address the complex challenges posed by climate change. Assessment methodologies are categorized into three primary approaches: expert experience, multi-criteria decision analysis, modeling and simulations. The most frequently employed assessment indicators include roof slope, roof area, stormwater retention, building height, temperature regulation, green space, building function, and roof material. The analysis indicates a lack of standardized protocols persists across indicator selection and methodological frameworks, limiting the comparability and transferability of findings. We recommend the development of standardized assessment protocols, climate-based classification systems, and data-driven approaches for indicator selection. Additionally, an integrated framework incorporating standardized assessment criteria is essential to facilitate cross-regional comparisons. This review contributes to the growing body of knowledge and development on GR field and provides guidance for the proposal of general GRRP assessment guidelines to better, support GR spatial planning in high-density built environments.