This article analyses participants’ reasoning for their city’s membership in transnational municipal networks and the extent to which this changes over time. Theoretically, we build on new-institutional theory and conclude that although parts of the members’ reasoning have rational components, a discursive institutional perspective improves the understanding of cities’ membership of transnational municipal networks. This perspective uncovers how important aspects of transnational municipal network participation are motivated by a different logic than that of measurable output. Cities use transnational municipal networks as sources of internal and external legitimacy, to legitimatise their position in domestic politics and their international position among other ‘global’ cities.