A growing literature explores how IOs legitimize themselves in the face of contestation. Yet we know less about the extent to which these patterns are reproduced – or rejected - by national politicians. This article therefore asks: to what extent, and on what grounds, do national parliamentarians legitimate and delegitimate IOs? We use LLMs to capture both the stance and legitimation patterns used by MPs toward 75 different IOs between 1990 and 2018 (n > 600,000). We show how (de)legitimation patterns differ based on the level of authority of IOs, a parliamentarian’s governing status and between Global South and Global North IOs. Importantly, we find that national parliamentarians legitimize IOs based almost exclusively on performance (output), suggesting IOs’ increasing focus on procedural based legitimation is not mirrored at the national level.