We describe the finding of an urban roost of wintering Barn Swallows Hirundo rustica in Aveiro, Portugal, which remained stable with c.150 individuals from at least early December to mid-January 2015/16. The stability in numbers and the occurrence of freshly-shed primary feathers below the roost strongly indicate that these were indeed wintering birds that completed their annual moult and suffered a relatively low mortality during this period. The number of roosting Barn Swallows steeply declined in late January, coinciding with the appearance of singing males in the area and the arrival of numerous migrants from Africa to Portugal. The site characteristics and roosting behaviour are described, and the potential advantages of urban roosting discussed. Although this particular winter was unusually mild, observations indicate that this roost may have been present in previous years, and others may occur elsewhere in the Iberian Peninsula.