ABSTRACT This article studies the politics of regional cooperation and regionalism, with particular attention to “good neighbourly relations”, which has emerged as a new concept in the field. Since the early 1990s, the Balkan countries have pursued attempts to link with each other in regional platforms and networks. Applying the concept of “good neighbourly relations”, this article seeks to reveal how the countries of the region have pursued regional cooperation while facing the reality of persistent bilateral disputes among themselves. While influential studies point to a growing cleavage among neighboring countries, this article seeks to further the understanding of regional cooperation with an assessment of the emergence and rise of the concept of “good neighbourly relations” to a norm of regionalism. By applying a social constructivist perspective this article investigates whether, and how, “good neighbourly relations” functioned as a channel of diffusion of norms pertaining to regionalism and regional cooperation in the Balkans. The article suggests that “good neighbourly relations” provided an opportunity for norm diffusion. By allowing the emerging norm of “good neighbourly relations” to guide regionalism efforts, the ambitions to promote the norm regionally as well as globally were supported.