The present study describes antihyperglycaemic activity in the two soft corals Sinularia firma Tixier Durivault and Sinularia erecta Tixier Durivault (phylum: Coelentrata). The methanolic extracts of S. firma and S. erecta exert a blood-glucose-lowering effect of 14.5% and 16.1%, respectively, on streptozotocin (STZ)-induced diabetic rats at an oral dose of 250 mg/kg body weight. These extracts were also found to inhibit the postprandial increase in hyperglycemia in normal rats by 13.0% and 12.7%, respectively, post sucrose challenge. Many compounds were isolated from these extracts; however none of them have shown a promising inhibitory effect on the tested enzymes of the insulin/glucose/glycogen cascade, i.e., protein tyrosine phosphatase-1B, glucose-6-phosphatase, and glycogen phosphorylase, respectively.