The thymus is central to the adaptive immune system, but it is one of the first organs to undergo an age-related decline in function. Reduced expression of the thymic epithelial cell (TEC)-specific transcription factor FOXN1 has been associated with thymus degeneration, but whether restoration of FOXN1 expression can regenerate an aged thymus is unknown. Now, on p. 1627, Clare Blackburn and colleagues show that provision of FOXN1 in the thymus can reverse fully established age-related thymic degeneration. The authors use an elegant transgenic mouse model to induce the expression of FOXN1 exclusively in the TECs of aged mice, and show that the resulting rejuvenated thymus displays tissue architecture and gene expression similar to that of a much younger mouse. Importantly, the regenerated thymus can generate and export new T cells: a function that is crucial for its role in the adaptive immune system. This is the first report of the regeneration of a whole, aged organ by a single factor and has exciting implications for regenerative medicine.