In July 2023, the International Maritime Organization (IMO) resolved to reduce international shipping’s greenhouse gas emissions to net zero “by or around, i.e., close to” 2050. There is a long-running debate about whether the sector should decarbonize and how it could do so in a way that is equitable for states and the shipping industry. This Article is the first to normatively define shipping’s fair share of the overall climate mitigation burden using principles of international environmental law. It refers to the IMO’s institutional rules and practice to identify relevant principles, evaluates emission reduction pathways based on the sector’s technological potential, and determines that its fair share would be its highest possible ambition in light of its unique capacity to mitigate. The Article ties shipping’s climate goals to a framework of international environmental law, and offers a structure to assess its ambition going forward.