This research studies norm subsidiarity in the practice of diplomacy. Norm subsidiarity is a concept found within norm diffusion scholarship. It is argued that this scholarship gives a privileged position to materially strong actors in its explanation of norm diffusion. The role of weaker states to support international norms is often ignored. Norm subsidiarity explains how materially weaker states support international norms to protect their autonomy from stronger actors. This research demonstrates the application of norm subsidiarity by tracing the history and context surrounding the diplomacy of Iran’s nuclear programme from its inception in 1957 to the present. Iran has successfully supported the norm of the peaceful use of nuclear energy by continuing to develop its nuclear programme. This is despite overwhelming international pressure from much stronger states who seek to stop this programme. The analysis shows that Iran’s actions can be described as a process of norm subsidiarity. This provides credibility to the argument that weaker states can play an important role in successfully supporting international norms, despite the desires of much stronger states.