Gestational diabetes (GD) is a common complication during pregnancy. Metabolic changes in GD affect fetal development and fetal glucose homeostasis. The present study utilized a rat model of GD to evaluate the effects of nicotinamide on diabetic parameters; antioxidant gene expression viz, superoxide dismutase (SOD) and catalase (CAT); reactive oxygen species (ROS) production by neutrophils and enhancement of lymphocyte mediated immune response. Nicotinamide (50, 100 and 200 mg/kg) was orally supplemented to gestational diabetic rats from days 6 through 20 of gestation. After GD induction, the control group had elevated glucose and reduced insulin while nicotinamide (100 & 200 mg/kg) supplementation reversed these changes. The same doses of nicotinamide upregulated mRNA expressions of SOD and CAT genes in liver but reduced the oxidative burst activity of neutrophils in response to phorbol myristate acetate (PMA), N-formyl-methionyl-leucyl-phenylalanine (FMLP) or E. coli activation. Nicotinamide (100 & 200 mg/kg) supplementation also increased expression of activated T helper (CD4+CD25+) cells and induced proliferation of splenocytes. These findings provide evidence for utilizing nicotinamide as supplement or adjunct to support existing therapeutic agents for gestational diabetes and in pregnant individuals with weakened immune systems.