Sustainability within planetary boundaries requires concerted action by individuals, governments, civil society and private actors. For the private sector, there is concern that the power exercised by transnational corporations generates, and is even central to, global environmental change. Here, we ask under which conditions transnational corporations could either hinder or promote a global shift towards sustainability. We show that a handful of transnational corporations have become a major force shaping the global intertwined system of people and planet. Transnational corporations in agriculture, forestry, seafood, cement, minerals, and fossil energy cause environmental impacts and possess the ability to influence critical functions of the biosphere. We review evidence of current practices and identify six observed features of change towards 'Corporate Biosphere Stewardship', with significant potential for upscaling. Actions by transnational corporations, if combined with effective public policies and improved governmental regulations, could substantially accelerate sustainability efforts. MAIN TEXT Consolidation among corporations, whereby a small number of companies control a large market share of the overall output or sales for a particular product or product type (i.e. oligopoly or, at the extreme, monopoly), is a well-known 1,2 and predictable 3,4 feature of economic development 5. Some 10% of the world's corporations generate 80% of all profits globally 6. A handful of transnational companies (TNCs) in the information technology sector control 90% or more of the global market share of search engines, operating systems, and social media 7. Three investor firms manage over 90% of all assets under management in passive equity funds 8 , and retailers, which form the interface between consumers and global supply chains, also show high levels of concentration 9,10. Such dominance is variously explained by increasing share of returns from growth going to capital rather than labour, the ability of TNCs to navigate regulatory systems opportunistically across multiple jurisdictions, and their capacity to create barriers to entry for smaller firms 11 .