Morinda citrifolia Linn (Rubiaceae) is a traditional medicinal herb that has been purported to be beneficial in the treatment of infections due to its immune enhancing properties. However, detailed studies highlighting the effect of different compounds isolated from the plant on the immune system are lacking. In this study, the stimulatory effects of the extracts and fractions of M. citrifolia fruits on important components of the adaptive immune system such as T lymphocytes and B lymphocytes were studied. The effects of the plant extracts on lymphocytes were assessed by in vitro (MTT assay) and in vivo (cell mediated immune response) techniques. Results of the MTT study indicated that the hydroalcoholic (0.5 and 1.0 mg/mL) and aqueous extracts (0.5 and 1.0 mg/mL) significantly (p < 0.05) increased in vitro splenocyte proliferation to the extent of 43.6, 54.5, 32.7, and 36.4%, respectively. Moreover, the hydroalcoholic (200 mg/kg) and the aqueous (200 mg/kg) extracts significantly (p < 0.05) increased the cell-mediated immune response to the extent of 33.52 and 18.56%, respectively. The fractions F I, F II, and F III failed to elicit a significant stimulatory effect on lymphocytes in the in vitro and in vivo studies. The effect of the extractives of M. citrifolia fruits on B-cells was measured by the delayed type hypersensitivity method. The study revealed that the hydroalcoholic extract (200 mg/kg) and fraction F I (40 mg/kg) significantly increased the humoral response to the extent of 33.33 and 35.12%, respectively. The results of this study confirm the cellular and humoral immunostimulant properties of M. citrifolia fruits and justify its usage in traditional medicine.