Schizophrenia (SCZ) and bipolar disorder are severe mental disorders, both placing a significant burden on individuals’ wellbeing and global health generally. The complex interaction of multiple mechanisms, underlying these disorders, still needs further elucidation. Increased activation of components of the immune system may be involved, including alterations in intestinal permeability and gut microbiome. Probiotics, defined as living microorganisms conferring health benefits to the host when administered in adequate amounts, seem to have supportive therapeutic effect in psychiatric disorders. The authors in this review provide an overview of this emerging research field and summarize both the publicated microbiome studies in SCZ and bipolar disorder and the current clinical research using probiotic supplementation in patients diagnosed with these disorders.