Immunomodulatory activity of aqueous extract of Trigonella foenum graecum L., a widely used medicinal and dietary herb, was evaluated in male Swiss albino mice. Mice were treated with three doses of extract (50, 100 and 250 mg/kg body weight per os) for 10 days. Body weight, relative organ weight, cellularity of lymphoid organs, delayed type of hypersensitivity (DTH) response, plaque-forming cell (PFC) assay, haemagglutination titre (HT), quantitative haemolysis of SRBC (QHS) assay, phagocytosis, and lymphoproliferation were studied in various groups of animals. At doses of 50 and 100 mg/kg, a significant increase (p < 0.05) in relative organ weight of thymus was observed but there was no effect on kidney and spleen weights. Liver weight also increased significantly at doses of 100 and 250 mg/kg. However, no elevation in the levels of liver function test (LFT) enzymes was observed. As regards lymphoid organ cellularity, spleen recorded no significant increase at any dose, whereas cellularities of thymus and bone marrow were significantly increased. T. foenum graecum extract elicited a significant (p < 0.001) increase in the DTH response at doses of 50 and 100 mg/kg, but the change at higher dose of 250 mg/kg was not statistically significant. Humoral immunity as measured by PFC showed an elevated response at a dose of 100 mg/kg, but at 50 and 250 mg/kg, no significant effect was observed. In the HT test, plant extract also showed modulatory effect at all the doses. Plant extract elicited a significant increase in phagocytic index and phagocytic capacity of macrophages. Stimulatory response of plant extract was also observed in lymphoproliferation assay but the response was weak. Overall, T. foenum graecum showed a stimulatory effect on immune functions in mice. As it is used for a variety of medicinal purposes, its immunostimulatory effect, as reported in this study, strengthens the rationale of its use in several Ayurvedic and Unani drugs.