Group-living in carnivores is mostly associated with cooperative hunting and anti-predator defense. Giant otters (Pteronura brasiliensis) live in monogamous and cooperative breeding groups, where mechanisms other than cooperative foraging may be driving group maintenance in the species. We herein describe three interactions between giant otters and jaguars (Panthera onca) observed in the wild, two of which involved groups of otters and one, a lone individual. In the two group instances, the otters mobbed the jaguar until it left the area. The mobbing behavior displayed in these instances likely reinforces the advantages of living in groups, reducing predation risk and promoting group cohesion, with resulting territorial and fitness benefits.