Among the extant endemic carnivorans of Madagascar (family Eupleridae), the largest species is the fosa,Cryptoprocta ferox. A larger species, Cryptoprocta spelea, survived into the Late Holocene. Here, we report the discovery ofa morphologically distinct partial humerus from Mitoho in Tsimanampesotse National Park, southwestern Madagascar,belonging to a larger extinct form of Cryptoprocta that existed prior to 8.3 ka. We review the history of the genus, describethis new taxon, and briefly describe the flooded cave system where it was found. We reconstruct body size for Cryptoproctasubfossils from Mitoho and other sites and document geographic variation in C. spelea across Madagascar. While the MitohoC. spelea specimens are larger than those found elsewhere, the newly described humerus is even larger and cannot beattributed to a male C. spelea. Functional analyses suggest all species of Cryptoprocta, irrespective of body size, had arborealadaptations and that the new species had enhanced strength associated with climbing or prey capture. Subfossils of all threespecies were found on the same submerged ledge at Mitoho, raising the possibility of sympatry in southwestern Madagascar.Regional stable carbon isotope analyses indicate that C. spelea preyed upon larger lemurs than those typically targeted byextant C. ferox. Body size and morphology suggest the new species targeted even larger prey. Collectively, these findingssuggest that the Holocene Malagasy carnivoran guild exploited substantially larger lemur prey than do living euplerids,consistent with carnivore damage documented on remains of some of Madagascar’s largest extinct lemurs.