Introduction: The study evaluated the ameliorative effects of Helianthus annuus leaf extract on nephrotoxicity, cardiac, and haematologic disorders in alloxan-induced hyperglycaemic rats. Material and Methods: The cold maceration method with 80% methanol was used in the preparation of H. annuus extract. Thirty alloxan-induced hyperglycaemic rats were randomly assigned to five equal groups (A-E). Groups A and B received 5% tween-20 solution in water (5 mL/kg) and glibenclamide (2 mg/kg), respectively; while groups C, D, and E received 150, 300, and 600 mg/kg of the extract, respectively, per os once daily for 21 consecutive days. The levels of serum urea, creatinine, haematological indices, and histopathological changes in the kidneys and heart were evaluated 24 h after the last treatment on day 21. Results: The extract and glibenclamide significantly (P < 0.05) reduced the levels of serum urea and urea : creatinine ratio in diabetic rats when compared with the vehicle treated group. The extract and glibenclamide also ameliorated haematological disorders and kidney and cardiac damage induced by alloxan. Conclusion: H. annuus extract produced nephroprotective, cardioprotective, and haematoprotective effects and might prevent the advancement of diabetic complications such as diabetic nephropathy and cardiovascular diseases in diabetic patients.