An array of brain regions in the fronto-parietal and temporal lobes cooperates to process observation and execution of actions performed by other individuals. Using functional MRI, we hypothesized that vegetarians and vegans might show brain responses to mouth actions performed by humans, monkeys, and pigs different from omnivores. We scanned 20 omnivores, 19 vegetarians, and 21 vegans while watching a series of silent videos, which presented a single mouth action performed by a human, a monkey, and a pig. Compared to omnivores, vegetarians and vegans have increased functional connectivity between regions of the fronto-parietal and temporal lobes versus the cerebellum during observation of mouth actions performed by humans and, to the same degree, animals. Vegans also had increased connectivity with the supplementary motor area. During human mouth actions, increased amygdala activity in vegetarians and vegans was found. More critically, vegetarians recruited the right middle frontal gyrus and insula, which are involved in social mirroring, whereas vegans activated the left inferior frontal gyrus and middle temporal gyrus, which are part of the mirror neuron system. Monkey mouth actions triggered language network activity in both groups, which might be due to the attempt to decode monkey mouth gesture, with an additional recruitment of associative temporo-occipital areas in vegans, whereas pig mouth actions activated empathy-related regions, including the anterior cingulum. These results support the role of the action observation-execution matching system in social cognition, which enables us to interact not only with our conspecifics but also with species in phylogenetic proximity to humans.