Complex problems and the pursuit of Sustainable Development Goals (SDGs) in today’s interconnected world often exceed the capacity of traditional methods, requiring approaches that engage all of society, incorporate diverse perspectives, and foster legitimacy. The Crowdsourcing Delphi methodology is proposed as a hybrid approach, combining the structured expert opinion of the Delphi method with lay insights and the wisdom of crowds via crowdsourcing. Designed for forward-looking, complex problem-solving, it enables iterative collaboration between experts and the wider public, enhancing both analytical rigour and creative insight. Aligned with the Double Diamond model, it links divergent and convergent thinking phases to guide exploration and synthesis. Comparative analysis and practical examples demonstrate that the Crowdsourcing Delphi methodology provides a systematic yet flexible framework for tackling complex problems in policy, innovation, and sustainable development. By fostering democratic deliberation and broad societal engagement, the Crowdsourcing Delphi methodology advances inclusivity, legitimacy, and practical impact, emphasising its novelty as a methodology for addressing contemporary global challenges.