Digital labour platforms are contested because they create work relationships that neither fall into the category of traditional employment nor fulfil the criteria of freelance work due to platforms’ heavy interference in the work process. Although existing research has elucidated different pathways for improving the working conditions of platform workers, we develop an employment relations perspective on platform-mediated work systems building on David Marsden's theory of employment systems. On the basis of an assessment of the psychological, economic and legal contracts underlying platform-mediated work, we discuss the limited functionality of these contracts in their ability to control opportunism among platforms, workers and clients. Our analysis contributes to existing debates by providing a theoretically informed framework for analysing the design of platform-mediated work systems and their consequences for the participating parties. Our insights add nuance to the platform regulation debate by going beyond the question of worker classification towards understanding the organizational and institutional elements of the work system that need to be redesigned and supported by appropriate institutional safeguards. An employment systems perspective also allows for reflection on the economic, societal, sectoral and strategic conditions that could drive such changes.