The highly diverse Polysphincta genus group includes a large number of species whose host spider is unknown. Here, we describe the first record of a species of Acrotaphus parasitizing an araneid in the genus Acanthepeira, in this case a subadult male. The cocoon web that the male spider constructed, induced by the third instar wasp larva, consisted of at least seven silk lines that merged into a single thick line 8 cm long. The larva constructed its cocoon near the middle of this thick line, from which an adult male wasp emerged twelve days later.