Green ammonia, made using renewable energy, is emerging as a low-carbon fertilizer, shipping fuel and hydrogen carrier, yet its governance remains fragmented across sectors. This paper outlines three concerns about its rapid expansion: nitrogen emissions that could erode its climate benefits, knock-on effects on fertilizer markets and food security, and the risks and opportunities of decentralized production. It proposes six principles for governing all forms of ammonia, spanning food security, equitable access, environmental safeguards, transparency, safety and adaptive governance, alongside practical guidance for vessel operators, ports, governments, international bodies, fertilizer producers and researchers.