Spiders are a diverse and abundant group of predaceous arthropods in arid environments. Spiders in the genus Eresus Walckenaer, 1805 are widely distributed in mesic and arid regions of the Palaearctic, but data on their diet are scarce. The goal of this study was to analyze the diet of Eresus kollari Rossi, 1846 in an arid habitat of the southeastern Iberian Peninsula. A total of 64 webs of Eresus kollari with prey remnants were collected in a field site at the Guadix-Baza Basin, and prey were identified to species or the lowest possible taxonomic level, and counted. Prey size was estimated based on remains from the webs and voucher specimens from the study area. In addition, laboratory observations of prey capture were made. The results showed that E. kollari has a broad diet, including prey from a total of 106 taxa. Prey included large arthropods (mostly tenebrionids) as well as relatively small insects (e.g., ants, which constituted a high proportion of prey). Laboratory observations showed that adult E. kollari actively captured tenebrionid beetles crawling on silk threads around the web, and ants moving on the surface of the web sheets. The high proportion of large-size prey in the webs studied, especially tenebrionids, indicate that this spider has a diet similar to that of Latrodectus lilianae Melic, 2000 in the study area, but the different hunting techniques used by the two spiders probably account for the dissimilar proportion of ants and predacious arthropods in the two diets.