Gestational diabetes mellitus (GDM) is a dangerous complication of pregnancy which is induced via dysfunction in glucose metabolism during pregnancy. Sinomenine (SM) has already proved an antidiabetic effect against streptozotocin (STZ)-induced diabetes mellitus (DM) in rats. In this protocol, we examined the protective effect of SM against STZ-induced GDM in rats. Wistar rats were divided into three groups and STZ (40 mg/kg) was used to induce GDM. At the end of the experimental protocol, bodyweight, pub weight, and survival rate were estimated. Blood glucose level (BGL), fasting insulin (FINS), free fatty acid (FFA), Hemoglobin A1C (HbA1c), and C-peptide were measured. Lipid, antioxidant, inflammatory cytokines, and inflammatory mediators were also determined. RT-PCR was used for estimation of the role of TLR4/ MyD88/NF-κB signaling pathway. SM treatment significantly (p < .001) reduced BGL, hepatic glycogen, and improved the levels of FINS, C-peptide, FFA, and HbA1c. SM significantly (p < .001) suppressed the levels of total cholesterol (TC), low-density lipoprotein (LDL), triglycerides (TG), coronary artery index (CAI), very low-density lipoprotein (VLDL), atherogenic index (AI), and boosted high-density lipoprotein (HDL) levels. SM significantly (p < .001) decreased the lipid peroxidation (LPO) level and enhanced glutathione peroxidase (GPx), total antioxidant capacity (TAC), glutathione S-transferase (GST), superoxide dismutase (SOD), respectively. It reduced the levels of inflammatory cytokines including interleukin-1β (IL-1β), tumor necrosis factorα (TNFα), interleukin-6 (IL-6), and inflammatory mediators viz., nuclear kappa B factors (NF-κB). SM significantly (p < .001) reduced the mRNA expression of Myd88, NLRP3, TLR4, and NF-κB, which were boosted in the GDM group rats. These findings suggest that SM could be a probable drug to be used for treating GDM via inhibition of the TLR4 signaling pathway.