Piedras Blancas National Park, in southern Costa Rica, is an important component of two biological corridors connecting the Osa Peninsula (Corcovado National Park) and La Amistad International Park. Understanding the mammal community composition of Piedras Blancas will provide baseline data to evaluate the success of conservation efforts. We used camera traps and opportunistic observations to describe the medium-sized and large mammals of the park. We deployed camera traps for 1,440 trap nights (2016-2018). We detected 19 mammal species from seven orders and 13 families. Five species are globally threatened: Leopardus wiedii (Schinz, 1821), Saimiri oerstedii (Linnaeus, 1758), Ateles geoffroyi (Kuhl, 1820), Alouatta palliata (Gray, 1849), and Tapirus bairdii (Gill, 1865). We did not detect two locally threatened species, Panthera onca (Linnaeus, 1758) and Tayassu pecari (Link, 1795). Our research highlights a need for critical conservation work within the proposed biological corridor to support Costa Rica’s most threatened wildlife.