This article examines the role of transboundary trust throughout the evolution of relations regarding the Swedish Barseb€ ack nuclear power plant, located opposite Copenhagen on the narrow straits separating Denmark and Sweden, which created cross-border tensions until its closure in 1999/2005. The article examines changes in transboundary trust in and between the actors relevant for decision-making regarding the plant, during the three phases of the conflict. It focuses on three aspects, which shaped the development of transboundary trust: first, the role played by pre-existing structures of cross-border institutional, interpersonal and ideological trust, secondly, practices of trust-building that various actors applied to maintain, enhance or rebuild transboundary trust, and thirdly, the growing discourses of mistrust and even distrust in the institutions advocating the use of nuclear powerregardless on which side of the border they were located. The article argues that changes in contemporary views on nuclear risk and the concomitant politicisation of the issue of nuclear power broadened the range of relevant actors, and contributed to changing trust relations. It suggests a mutually reinforcing interaction between interpersonal, institutional and ideological trust, and finds few differences in discourses about institutional trust between transboundary and domestic authorities. Its findings highlight the importance of productive mistrust for nuclear safety, and thus demonstrate that both trust and mistrust are important for living with, but also for limiting (nuclear) risk.