Tribulus terrestris L (TT) is used in the Arabic folk medicine to treat various diseases. The aim of this article was to investigate the protective effects of TT in diabetes mellitus (DM). Diabetes is known to increase reactive oxygen species (ROS) level that subsequently contributes to the pathogenesis of diabetes. Rats were divided into six groups and treated with either saline, glibenclamide (Glib), or TT for 30 days. Rats in group 1 were given saline after the onset of streptozotocin (STZ)-induced diabetes; the second diabetic group was administered Glib (10 mg/kg body weight). The third diabetic group was treated with the TT extract (2 g/kg body weight), while the first, second, and third nondiabetic groups were treated with saline solution, Glib, and TT extract, respectively. At the end of the experiment, serum and liver samples were collected for biochemical and morphological analysis. Levels of serum alanine aminotransferase (ALT) and creatinine were estimated. In addition, levels of malondialdehyde (MDA) and reduced glutathione (GSH) were assayed in the liver. The tested TT extract significantly decreased the levels of ALT and creatinine in the serum (P < 0.05) in diabetic groups and lowered the MDA level in liver (P < 0.05) in diabetic and (P < 0.01) nondiabetic groups. On the other hand, levels of reduced GSH in liver were significantly increased (P < 0.01) in diabetic rats treated with TT. Histopathological examination revealed significant recovery of liver in herb-treated rats. This investigation suggests that the protective effect of TT for STZ-induced diabetic rats may be mediated by inhibiting oxidative stress.