This article proposes a re-theorisation of the main social relations of platform work, based on two concepts drawn from Marx: subsumption of labour and the cash nexus. Platform work research to date is heavily empirical in character, with little theoretical development. As a result, the social relations of platform work are treated descriptively, using ad hoc or common-sense categories, or platforms’ own terminology. This under-theorisation leads to over-estimation of platform work’s novelty, decentring of capital in accounts of its development, incipient technological determinism and problematic generalisation from emergent trends. In place of the commonly assumed ‘triangle’ of platform work relations, this article argues that platform work is best understood in terms of an emerging labour–capital relation, which establishes a cash nexus between platform and worker as a result of a process of subsumption. This re-theorisation, in turn, helps to understand the rapid emergence of platform worker organisation and resistance, and the similarity of its demands with worker resistance in other, more established areas of paid work under capitalist relations of production.