Across mammalian species, social interaction with peers is considered a rewarding experience. In rats, social behaviours during adolescence are of principal importance for the development of appropriate adult behaviours. Social behaviour in rats has been shown to be sensitive to environmental and neurochemical factors such as stress and monoamine signaling in the prefrontal cortex and hippocampus. Palatable food consumption activates the mesocorticolimbic reward pathway in the brain, and overconsumption of these foods is detrimental to aspects of learning and behaviour mediated by the hippocampus and prefrontal cortex. In these experiments we examined the impact of 2 h daily access to a high fat / high sugar (HFHS) diet on social behaviours in adolescent male rats. Social interaction behaviours with a novel control rat were reduced when HFHS consuming rats were withdrawn from the palatable diet, but not following access to the HFHS food. Rats exposed to the HFHS diets also showed impairments at a social memory task requiring them to discriminate between a novel and a familiar rat. However, HFHS rats showed similar object recognition memory and odour recognition memory to control chow fed animals suggesting other aspects of memory were not impaired. Analysis of gene expression in the prefrontal cortex and hippocampus by qPCR demonstrated reduced levels of monoamine oxidase A (MAO-A) and catechol-O-methyltransferase (COMT) mRNA in HFHS diet fed rats compared to controls. These genes are involved in the synthesis of monoamines including dopamine and serotonin, indicating that the HFHS diet had an impact on reward neurotransmission in these brain regions. Furthermore, we observed significant differences in gut microbiota composition between the diet groups, particularly with respect to decreased abundance of Bacteroidetes, which has been linked to obesity and mood disorders in humans.