Seven species of Baeus occur in the Neotropical and Andean regions: B. achaearaneus Loiácono, B. auraticeps Girault, B. kuscheli Ogloblin, B. latrodecti Dozier, B. metazygiae Loiácono et Margaría, B. platensis (Bréthes), and B. ventricosus Ogloblin. All of them are associated with spiders of the families Araneidae and Theridiidae. In this study, a new species of Baeus is described from egg sacs of Cyclosa morretes (Araneidae) collected in a natural reserve in São Paulo State, Brazil. The spider species is a new host for Baeus. We also evaluated the frequency of attacks and the impact of parasitism on the spider egg sacs.