Although relatively rare among harvestmen in the superfamily Gonyleptoidea, paternal care has been observed in the families Manaosbiidae and Gonyleptidae, but not previously in the Cosmetidae. In this study, we describe multiple observations of egg guarding by adult males of an undescribed species of cosmetid harvestman from Volcán Cacao, Guanacaste Province, Costa Rica. Observations were made from 26–28 July 2010, during the wet season. In this species, males only guard eggs after dusk, leaving eggs unattended during the day. Based upon differences in color and size, males guarded eggs through several stages of development. When guarding, males contacted the first two pairs of legs with the eggs. Oviposition sites consisted of the undersides of leaves of small plants, with eggs closely packed together in a single layer covered by abundant, transparent mucus. The largest, darkest eggs were located near the distal tip of the leaf.