Our previous studies have shown that Gasca D™; a polyherbal formulation made from three well known antidiabetic plants in Nigeria, possess anti-hyperglycemic, body weight and blood lipid lowering potentials in rats. In view of this, the present study was therefore designed to evaluate the complementary therapeutic efficacy of Gasca D™ supplementation with lifestyle modification in a clinical setting. The randomized cohort trial was conducted in sixty (60) newly diagnosed type 2 diabetic patients on life style modification as first-line therapy who willingly volunteered to participate. The subjects that satisfy our inclusion criteria were randomly assigned into Gasca D™ (GG) and lifestyle (LG) groups. The GG group received 2000 mg Gasca D™ capsules daily over a twelve weeks period whereas the LG group received none. Biochemical estimates were conducted on subjects to evaluate the safety and efficacy of the polyherbal supplement at the end of the intervention period. Mean fasting blood sugar (FBS) showed a decrease of 23.4% (p < 0.05) in the GG group contrary to a 9.6% mean FBS observed in the LG (p < 0.05) group. Variation in glycosylated hemoglobin (HbA1c) was in line with that of FBS. Safety of Gasca D™ was demonstrated by a normal renal and liver function profile in both groups. Our finding suggests that, Gasca D™ can successfully be used as an effective and safe complementary nutraceutical for the control of hyperglycemia in type 2 diabetic patients on life style modification.