We assessed the effects of Zizyphus lotus L. (Desf.) polyphenols (ZLP) on T-cell signaling and proliferation. Our results showed that ZLP exerted no effect on the increases in intracellular free calcium concentrations, [Ca]i, in human Jurkat T-cells. However, ZLP modulated the thapsigargin-induced increases in [Ca]i in these cells. ZLP treatment was found to decrease the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2). In addition, ZLP induced a rapid (t1/2 = 33 s) and dose-dependent decrease in intracellular pH (pHi) in human Jurkat T-cells. Furthermore, ZLP significantly curtailed T-cell proliferation by diminishing their progression from S to G2/M phase of cell cycle, and the expression of interleukin-2 (IL-2) mRNA. Taken together, the results of the present study demonstrate that ZLP modulate cell signaling and exert immunosuppressive effects in human T-cells. HIGHLIGHTS: ▪ Zizyphus lotus polyphenols (ZLP) modulate Jurkat cell activation at the molecular level. ▪ The ZLP altered increases in [Ca]i, evoked by thapsigargin. ▪ The ZLP also induced a decrease in the intracellular pH in Jurkat cells. ▪ The ZLP blocked T-cell cycle progression and decreased MAP kinase phosphorylation.