The influence of non-governmental organizations (NGOs) on international organizations is a perennial question in global governance research. We examine whether and why horizontal venue-shopping strengthens NGO influence on international organizations in the area of biodiversity protection. We argue that horizontal venue-shopping facilitates NGO influence, building on previous studies on institutional complexity and NGO influence in global governance. The argument is examined through process tracing based on extensive fieldwork material in the context of the governance of sharks in the International Commission for the Conservation of Atlantic Tunas during 1994–2021. We find NGO preference attainment in a majority of the studied policy processes, and indicative evidence of increased NGO influence over time. The results suggest that horizontal venue-shopping strengthened the influence of NGOs in several of the examined processes. These findings have broader implications for research on NGO influence in an increasingly complex global governance landscape.