The Miocene deposits of South America are notable for their diverse crocodyliform fauna, of which the giant caimanine Purussaurus is a well-known example. This contribution describes a new caimanine, Acresuchus pachytemporalis, gen. et sp. nov., based on an almost complete skull and mandible from the late Miocene Solimões Formation of the southwestern Brazilian Amazonia. This new taxon is based on a unique combination of characters, of which the presence of an upturned posterolateral margin of the squamosal throughout the entire lateral margin of the bone (a ‘horn’), with a dorsoventral expansion in the posterior portion of the eminence, stands out. We conducted a phylogenetic analysis of Eusuchia, which showed the new taxon as sister to Purussaurus. This placement allows discussion about the evolution of gigantism in the Acresuchus-Purussaurus clade, which reveals several characters that may be related to gigantism. Additionally, Acresuchus was probably a medium-sized generalist caimanine that had an ecological niche similar to the extant Melanosuchus niger. Until now, crocodyliforms that had such niches were unknown from the Solimões Formation.