Sustaining our planetary future requires a world with clean water, affordable energy, reduced carbon and methane emissions, reduced habitat loss, eradication of poverty, reduction of global inequalities, and quality — as well as equitable — medical care and education. Achieving these endpoints requires innovation, systems thinking, and ultimately, mass human behavior change. This is attainable by fostering a clear scientific and economic evidence base, coordinated and diverse leadership, sustained motivation, and the right mindset and public messaging. Brain capital is a novel framework that recognizes brain skills and brain health as indispensable drivers of the modern knowledge economy. The concept of “green brain capital” — which we are scoping in this paper — places a central emphasis on the brain to deliver a healthy and sustainable environment and, vice versa, on a green environment to promote and safeguard brain health. The environmental determinants of brain health are foundational to this model, as brain health is key to navigating the modern world and thriving. Key brain skills include green skills, creativity, adaptability, digital literacy, and ecological intelligence. Green brain capital intends to be politically pragmatic, economically aware, and lifespan-focused. Growing green brain capital will require transformations across hierarchical levels in social-ecological systems, ranging from individuals to sub-populations to entire societies.