The hypoglycaemic and anti‐hyperglycaemic potential of a methanolic extract of Jussiaea suffruticosa Linn. (family‐Onagraceae) was evaluated in normal and alloxan‐diabetic rats. In normal rats, the methanolic extract of J. suffruticosa Linn. (200 and 400 mg kg−1) exhibited significant hypoglycaemic activity (P < 0.05) within 3 h after oral administration. The plasma glucose concentration was 89.83±6 mg/100 mL (control 115.5±3.6 mg/100 mL) at 3h and 78.16±5.7 mg/100 mL (control 113.2±4.2 mg/100 mL) at 4.5 h after oral administration of 400 mg kg−1 of the methanolic extract. The effect was prolonged up to 24 h and the plasma glucose concentration fell to 75.66±5.5 mg/100 mL (control 112.16±3.7 mg/100 mL). The effect was dose‐dependent. In alloxan‐diabetic rats, the methanolic extract (400 mg kg−1) showed a significant anti‐hyperglycaemic effect (P < 0.05) from 1.5 h after oral administration. This effect was prolonged up to 24 h and the plasma glucose concentration fell to 173.66±6 mg/100 mL (control 296.67±4.9 mg/100 mL) compared with 166.83±6.7 after oral administration of 10 mg kg−1 glibenclamide (standard). The results indicate that the methanolic extract of whole plants of J. suffruticosa Linn. has potent hypoglycaemic potential in normoglycaemic rats and alloxan‐diabetic rats.