Informal rental housing remains a hidden – yet central – pillar of low-cost shelter markets across the global South. The large informal settlement of Mathare Valley illustrates Nairobi’s rapidly changing informal rental housing markets, with the ongoing rise of tenements alongside metal shacks. Informal single-room rental units (shacks and tenements) already house most Nairobi residents in low-quality but highly profitable shelter. This case study describes how multiple exclusions can shape access to rental housing and examines the politics of shelter delivery. We underscore the importance of private rental markets and offer recommendations for inclusive, multi-pronged interventions combining support for rental housing, land governance and infrastructure delivery. Through a better understanding of the myriad actors and dynamics of informal rental housing, it may be possible to develop strategies that serve low-income tenants for whom renting is often the only viable option.