Diabetes Mellitus (DM) is associated with an increased susceptibility to various infections, which might be attributed to changes in immune response owing to chronic hyperglycemia. Nucleoside triphosphate diphosphohydrolase (NTPDase), 5′-nucleotidase, and adenosine deaminase (ADA) are important enzymes involved in the generation of anti-aggregant and anti-inflammatory microenvironments. The aim of this study was to evaluate the effect of gallic acid (GA) on the hematological parameters and ectonucleotidase activities in platelets, lymphocytes, and serum of diabetic rats. Experimental rats were categorized into 4 groups: (i) control -saline, (ii) control - GA, (iii) diabetic -saline, and (iv) diabetic - GA. One week after induction of DM using streptozotocin (65 mg/kg), GA (30 mg/kg) or saline was orally administered to the rats for 21 days. Our results demonstrated that the concentration of mean corpuscular hemoglobin was decreased, whereas that of red cell distribution was increased in the diabetic group, however, GA could revert these alterations. Moreover, in diabetic rats, GA reverted the increase in ATP and ADP hydrolysis and ADA activity in lymphocytes, and it prevented the increase in NTPDase and ADA activities in platelets. A decrease in ATP hydrolysis and an increase in ADP and AMP hydrolysis were observed in the serum of diabetic rats; however, GA treatment could solely revert changes in ATP hydrolysis. Our study suggests that GA exhibits beneficial effects on immuno- and thrombo-regulatory responses in DM and that these effects may be related to the modulation of purinergic signaling.