The neurotransmitter γ-aminobutyric acid (GABA) is an extracellular signaling molecule in the brain and in pancreatic islets. Here, we demonstrate that GABA regulates cytokine secretion from human peripheral blood mononuclear cells (PBMCs) and CD4+ T cells. In anti-CD3 stimulated PBMCs, GABA (100nM) inhibited release of 47 cytokines in cells from patients with type 1 diabetes (T1D), but only 16 cytokines in cells from nondiabetic (ND) individuals. CD4+ T cells from ND individuals were grouped into responder or non-responder T cells according to effects of GABA (100nM, 500nM) on the cell proliferation. In the responder T cells, GABA decreased proliferation, and inhibited secretion of 37 cytokines in a concentration-dependent manner. In the non-responder T cells, GABA modulated release of 8 cytokines. GABA concentrations in plasma from T1D patients and ND individuals were correlated with 10 cytokines where 7 were increased in plasma of T1D patients. GABA inhibited secretion of 5 of these cytokines from both T1D PBMCs and ND responder T cells. The results identify GABA as a potent regulator of both Th1- and Th2-type cytokine secretion from human PBMCs and CD4+ T cells where GABA generally decreases the secretion. •GABA regulates cytokine secretion in anti-CD3-stimulated peripheral blood mononuclear cells (PBMCs) and CD4+ T cells.•GABA inhibits secretion of 47 cytokines in PBMCs from type 1 diabetes patients.•GABA regulates secretion of pro- and anti-inflammatory cytokines in a concentration-dependent manner. GABA is a signal molecule in the brain, blood and pancreatic islets where it is secreted by the insulin-producing β cells. GABA has many roles in human islets including optimizing function and survival of β cells. Bhandage et al. now show that GABA is a potent regulator of secretion of both pro- and anti-inflammatory cytokines in stimulated immune cells. In type 1 diabetes the β-cell mass is diminished and thus the protective effect of GABA in the islets although not in blood. Targeting GABA signaling in diabetes mellitus is likely to be a part of the solution when curing diabetes.