Copyright © Millennium Economics Ltd 2020. This paper introduces the notion of 'cognitive' institution and discusses its relevance to institutional economics. Cognitive institutions are conceptually founded on the philosophy of mind notion of extended mind, broadened to also include the distinctly social, institutional, and normative dimensions. Cognitive institutions are defined as institutions that not just allow agents to perform certain cognitive processes in the social domain but, more importantly, without which some of the agents' cognitive processes would not exist or even be possible. The externalist point of view of the extended mind has already had some influence in institutional economics: Arthur Denzau and Douglass North first introduced the notion of institution understood in terms of 'shared mental models', and relatedly philosopher Andy Clark introduced the notion of 'scaffolding institution'. We discuss shared mental models and scaffolding institutions and go a step further by showing that the notion of cognitive institution can capture more fundamental and salient aspects of economic institutions. In particular, we focus on the market as an economic cognitive institution.