The upper Maastrichtian elasmosaurid plesiosaur Aristonectes quiriquinensis from central Chile is here redescribed following further preparation of the holotype. New cranial characters include a flattened skull with a caudal extension of the mandibular articulation far posterior to the occipital condyle; a very long, horizontal, and posteriorly oriented paroccipital process; mandible with restricted gape angle; and teeth oriented rostrally and not interlocking during occlusion. Postcranial features include the presence of very long extremities (ca. 3 m), whereas features of the caudal-most vertebrae suggest the presence of a horizontal caudal fin. Additionally, neuroanatomical features of Aristonectes are for the first time described based on a braincase cast. Integration of osteological features, anatomical comparisons, as well as taphonomic and environmental data strongly suggests that the foraging behavior of Aristonectes quiriquinensis incorporated a mix of engulfment and feeding from benthic sediments. This is a novelty among elasmosaurids, so far considered as mainly epipelagic feeders.