Primary freshwater crabs are an important component of the food web in aquatic ecosystems, but our knowledge about the role of these decapods as predators and as prey is far from complete. Here we report observations of the feeding habits of the pseudothelphusid crab Ptychophallus uncinatus Campos & Lemaitre, 1999, made in 2013 during exploratory observations after sunset in the dusk and darkness of the early evening within the Veragua Rainforest Research & Adventure Park, Limón Province, in the Atlantic drainage of Costa Rica. We observed a case of cannibalism where an adult P. uncinatus was feeding on a smaller crab. Furthermore, P. uncinatus was observed to prey on an insect larva, a frog, and a lizard on three separate occasions. Additionally, a spider of the family Ctenidae was discovered feeding on a specimen of P. uncinatus. These observations revealed novel information about the role of P. uncinatus as both a predator and as an item of prey in a Neotropical freshwater ecosystem.