As an established climate leader within the United Nations Framework Convention on Climate Change (UNFCCC), the European Union (EU) has in recent years turned a fresh gaze to climate action in international transport, specifically via the International Civil Aviation Organization (ICAO) and the International Maritime Organization (IMO). The EU has declared a desire to work in both fora in a way wholly consistent with its approach to the UNFCCC. However, institutional differences in these two specialised UN fora give rise to a negotiation environment largely populated by officials from ministries of transport, including at the EU level, with working methods and priorities potentially conflicting with those in the UNFCCC. This likely complicates the EU’s ability to act on climate issues in these fora in a way that is coherent with its action in the UNFCCC. The paper answers the research question “To what extent is the EU’s external climate action in ICAO and IMO coherent with its action in the UNFCCC?” It looks at the cases of ICAO CORSIA (2016) and the IMO Initial Strategy (2018) – two political agreements on climate action adopted in the respective fora. Based on official documents and semi-structured interviews with EU, EU member state, and third state officials, the paper finds that the EU’s external climate action in both fora was only somewhat coherent with its action in the UNFCCC, a far cry from the integrated climate diplomacy called for in EU Council conclusions. The paper also provides preliminary insight into factors shaping EU climate action outside the UNFCCC, while developing a detailed framework for assessing cross-forum coherence at the international level for fora dealing with overlapping issue areas.