In an era marked by pressing environmental challenges and the urgent need for sustainable development, we need open source now, more than ever. Open Source, with its collaborative ethos and transparent nature, has the potential to revolutionise the way we approach sustainability. This keynote explores the symbiotic relationship between Open Source and sustainability, highlighting how Open Source principles foster innovation, inclusivity, and scalability, while nurturing a global ecosystem of problem solvers. Hong Phuc will share inspiring examples from diverse domains, showcasing how Open Source empowers individuals, organisations, and entire communities to create impactful and sustainable solutions.