Nudge and co-design are gaining popularity as innovative approaches to solving similar policy problems. Nudge is an approach to public policy that changes the context in which decisions are presented to citizens in order to encourage a particular choice. Co-design uses creative and participatory methods to engage citizens, stakeholders and officials in an iterative process to respond to shared problems. Both nudge and co-design supposedly achieve more effective outcomes, address big societal problems, and, in contrast to traditional policy approaches, consider humansâ€™ actual behaviour in a real-world context. In practice, we see them emerging and even merging together, despite significant tensions and contradictions between them. We critically examine the use of the approaches as policy instruments and consider the instrument constituencies that support them. By comparing and contrasting the two concepts in scholarship for the first time, the article highlights the assumptions underpinning the use of both nudge and co-design, arguing that each approach has its own underlying philosophy and claims on knowledge and authority. We reflect on the implications for policy effectiveness, political trust, and subsequently on government legitimacy.