The human gut microbiome is evolving as an independent organ with multitudinous functions spanning from the localised gut to peripheral organs including the brain. The role of gut microbiota in brain and mental health is of great interest to scientist, general public, and industries, because the gut microbiota is emerging as a novel therapeutic target for common mental disorders. The gut microbiota can modulate biological pathways that are implicated in common mental disorders via the gut-brain-axis, which is a complex system that involves the autonomic and enteric nervous system, and the neuroendocrine and neuroimmune components of the central nervous system. The bacteria in our gut communicate with the brain through various mechanism, which includes vagus nerve, immune mediators and microbial metabolites, that influence central processes such as neurotransmission and behaviour. Mounting evidence shows that the gut microbiota is implicated in the pathophysiology of mental disorders with altered microbial composition observed in patients with depression compared to the healthy counterparts. Whilst the gut microbiota can influence brain and mental health, many extrinsic factors (e.g., drugs, exercise, smoking) can shape the composition and functions of the gut microbiota and diet is one the strong influencers of the microbiota composition. Diet and dietary components such as probiotics, prebiotics, act as key modulators of the microbiota composition and thereby have shown to impact brain and mental health. This has led to the development and establishment of the field of “Nutritional Psychiatry”, which unfolds promising therapeutic options for improving debilitating common mental disorders with modifiable lifestyle factors such as diet. However, given the nascent feature of the field of Nutritional Psychiatry more clinical data supported by mechanistic evidence are needed.