With the rise of non-executive forms of diplomacy, political scientists have increasingly focused on the role of parliaments as diplomatic actors. This article aims to deepen the study of European parliamentary diplomacy through an extensive case study of transatlantic parliamentary diplomacy (TPD) between the European Parliament and the US Congress during the Obama and Trump administrations (2009–21). It is hypothesised that the decrease in EU–US political alignment under the Trump administration resulted in a corresponding decrease in both the salience and positivity of TPD. Counter-intuitively, the data shows that the decrease in political alignment correlated with a higher quantity of TPD. However, a content analysis of the joint reports of the Transatlantic Legislators’ Dialogue indicates that the attitude of this TPD is found to have become increasingly negative.