This systematic review investigates why and how platform workers express voice in a context where institutional and organisational voice mechanisms and representation structures are lacking or absent. Platform workers have been restricted in their ability to formally unionise or collectively bargain, and the presence of a digital intermediary in the form of a platform organisation limits the scope of worker voice. In this paper, we identify and synthesise the motives for voice use by platform workers—namely mutual aid, organising, visibility, and confrontation, and unpack how these are realised through bottom‐up and independent voice channels that may potentially influence multiple stakeholders. The paper's core contribution lies in highlighting how the blurred employment boundaries of platform work structurally render labour power even more indeterminate, informing our conceptualisation of a ‘grassroots voice mechanism’, wherein the social relations of platform work and digital technologies convey worker voice beyond traditional organisational boundaries. We conclude with an agenda to guide future research centred heavily around the dynamics of platform work, the use of novel voice channels, and worker attitudes towards them.