This article focuses on trade unions’ positioning towards the introduction of a Universal Basic Income (UBI), which is an understudied topic in research on the political feasibility of a UBI. Unions are often considered the ‘guardians’ of welfare states, as they have long fought to establish, protect, and preserve workers’ social rights throughout the history of Western societies. They are key socio-economic actors who represent the interests and needs of their members in political negotiations, with the aim of strengthening their social protection. Yet, unions have been particularly either opposed or unclear about their opinion of a UBI. Research on this topic remains scarce and shows inconsistent results with respect to the opinions of union members and officials. This article aims to fill this gap by investigating the reasons why Belgian union officials and their members oppose or support the implementation of a UBI. We will apply a mixed-methods approach that consists of a quantitative part using data from a novel vignette experiment to determine which features of a UBI elicit the most support or opposition among members. The qualitative section will examine how union officials perceive UBI proposals and how they react to their members’ preferences. In this way, this article will contribute to the literature by unravelling the complexity of Belgian trade union positioning towards the introduction of a UBI.