This article develops and extends the idea of cosmopolitan solidarity to temporality through a case study of archival activism and participatory film-making. It examines mediated witnessing within the Italian online audiovisual archive Archivio delle memorie migranti, which documents and archives the experiences of contemporary migrants in Italy. The moral basis of Archivio delle memorie migranti is cosmopolitan solidarity, which is usually understood as a practice that crosses spatial and communal boundaries. However, the ethics of solidarity also bridges past, present and future generations. Through the case of Archivio delle memorie migranti, this article demonstrates the significance of temporality in the theorization of cosmopolitan solidarity.