Morinda citrifolia (noni) fruit juice has the potential to influence immune system function. This review discusses results from several human and animal studies that provide insight into the potential mechanisms of action by which noni juice exerts its immunomodulatory effects. Increased natural killer cell activity is a likely a major contributor to the improved health outcomes and increased survival times described in case reports and as observed in LLC and S180 tumor bearing mice. Increased interferon-gamma (IFN-γ) production is also an important mechanism of action through which noni improves immune function. IFN-γ promotes natural killer cell activity and phagocytosis, activities both seen in human and bovine studies as well as in rodents. Noni promotes regulatory cytokine expression, such as IL-2 which stimulates CD4+ T cell differentiation. Noni juice appears to influence this process via kinase 1/2 (ERK1/2) protein kinase B (Akt) and nuclear factor-kappa-beta signaling. As oxidative status is known to influence immune function, this review also discusses the notable antioxidant properties of noni juice that have been demonstrated in human trials.