Some wasp species use spiders as food resources, overcoming several anti-predator barriers that are exerted by spiders. Tromatobia ichneumonid wasps are spider egg predators that usually attack Araneidae species, although there are few records of predation on Clubionidae, Philodromidae, Linyphiidae, Tetragnathidae, and Theridiidae spiders. Here, we describe the interaction between Tromatobia sp. and Chrysso compressa, a subsocial theridiid spider that exhibits extended maternal care, in the Atlantic Forest of southeastern Brazil. We observed that the larva of Tromatobia sp. develop inside the egg sacs of C. compressa, preying on the entire egg mass and building cocoons that change the color and morphology of the egg sacs. Despite these structural modifications, we registered an adult female of C. compressa guarding and caring for the cocoons (attacked egg sac) of the predators as if they were offspring (non-attacked egg sac). To the best of our knowledge, this study represents the first record of Tromatobia preying on Chrysso eggs.