United Nations (UN) entities have repeatedly been asked to mainstream their support for South-South and triangular cooperation (SSTC). However, there is hardly any systematic evidence on whether and how they have done so. This article contributes to addressing this gap. The analysis focuses on organizational efforts over the last two decades to integrate SSTC support into institutional processes across the UN development system. It centers around a scorecard of fifteen UN entities that maps the levels and contours of their organizational focus on SSTC. In light of a highly diverse SSTC support landscape and the complex political dynamics behind mainstreaming efforts, the article discusses the way ahead for UN engagement with SSTC, including the potentials and challenges of a continued focus on mainstreaming itself.