Chronic inflammation of the central nervous system (CNS) is critical to the pathogenesis of neuropsychiatric disorders (NPDs) that affect the global population. Current therapeutics for NPDs are limited to relieving symptoms and induce many adverse effects. Therefore, the discovery of novel therapeutic agents from natural sources is urgently needed. Intriguingly, the immune responses of peripheral organs are closely linked through the molecular communication between resident and blood-borne cellular components, which shape the neuroinflammatory phenotypes of NPDs. Since the gut and spleen are the two largest immunological organs of the body, the brain-gut microbiome and brain-spleen axes have been implicated in the connection between the CNS and the peripheral immune system. Accordingly, it has been proposed that the local CNS inflammation observed in NPDs is regulated via the manipulation of the systemic immune system by targeting the gut and spleen. Additionally, the complexity of the signalling network underlying the communication between the CNS and the systemic immune system suggests a strong potential for treating NPDs through a polypharmacological approach. The close association between systemic immunity and the homeostasis of the CNS points to the concept of repurposing interventions for systemic immune disorders to treat NPDs. Notably, natural products represent a promising source of such effective compounds due to both their pharmacological potency and safety. This review discusses the complex implications of dysregulated systemic immunity mediated by the brain-spleen and brain-gut microbiome axes in NPDs, such as Alzheimer's disease, Parkinson's disease, schizophrenia and major depressive disorder. In addition, the potential of repurposing natural product-based bioactive compounds for treating NPDs via modulating systemic immune disorders is intensively discussed.