Extensive green roofs are recognised as being a suitable habitat for some specific plants and arthropods that are resistant to very extreme hot and dry conditions. Little is known about spider species visiting and living on green roofs. In 2018 and 2019, 29 green roofs in Basel and four roofs in Aarau (Switzerland) were sampled using pitfall traps during the entire growing season. In the sampling period, 18467 adult spiders were collected and identified into 123 species. Two species were collected for the first time in Switzerland: Erigone dentosa O. Pickard-Cambridge, 1894 and Runcinia grammica (C. L. Koch, 1837). According to the Red List of Germany (and especially Baden-Württemberg), 26 species are of special interest from a nature conservation perspective. On the other hand, four species are alien species (i.e. they originate from other continents), and 14 species are expanding their range northward. Green roofs may therefore harbour endangered species, but there is also the possibility that the newly invading species may become dominant. Only after many years or even decades will it be possible to judge whether green roofs are primarily substitutes for lost habitats or stepping-stones for new invaders.