Mental health disorders are a leading cause of the global health-related burden, with depressive and anxiety disorders the main contributors to this burden. While pharmacologic treatments exist for the treatment of anxiety and depression, up to 30% of people with depression do not respond to pharmacological therapy. There is a need therefore for additional methods to help individuals experiencing stress, depression and anxiety. The gut microbiota is the collection of individual micro-organisms including bacteria, viruses, fungi and protozoa that live in the human gastrointestinal tract. The gut, the microbiota and the brain communicate extensively with each other through direct neuronal transmission as well as indirectly via hormonal and immune signalling metabolites. This connection is called the microbiota-gut-brain axis (MGBA) and has a profound impact on mental health. The microbiota and the MGBA is strongly influenced by diet. The psychobiotic diet is a diet which is high in prebiotic and probiotic rich fermented foods which is designed to support the gut microbiota in order to exert positive effects on the brain and mental health via the MGBA while avoiding or reducing consumption of foods which have a negative effect on gut health such as sweets, processed meats, fast food and particularly ultra-processed foods. The effectiveness of the psychobiotic diet in the treatment of stress, anxiety and depression was investigated through critical evaluation of recent, peer-reviewed scientific literature. The psychobiotic diet itself is hugely under researched, with the results of only two research studies published to date investigating a psychobiotic diet or a diet high in prebiotic-rich fruits and vegetables. These demonstrate that a psychobiotic diet or a diet rich in prebiotics are an effective treatment for stress, anxiety and can improve mood. The majority of research to date has investigated the effect of psychobiotic supplementation with probiotics, prebiotics or synbiotics. There is strong evidence that psychobiotics can improve stress, depression and anxiety. Psychobiotics were found to improve mood in healthy individuals, individuals with mild to moderate distress and individuals with clinical depression. There is evidence for these effects using psychobiotics alone and in conjunction with anti-depressant therapy. There is strong evidence that psychobiotic bacterial strains can protect against worsening mental health in individuals experiencing chronic stress, this suggests that the consumption of psychobiotics can have a beneficial, mental health protective role. A wide range of variables can influence the effectiveness of psychobiotics, such as individuals’ activity levels, mental health at baseline, proclivity to healthy behaviours and genetic background. Further well-designed, large-scale, long-term research studies are required to provide greater evidence for and to quantify exactly the effectiveness of the psychobiotic diet in the treatment of stress and anxiety.