Summaryα‐Glucosidase inhibitors, which can inhibit α‐glucosidase to reduce the release of glucose after a mixed carbohydrate diet, has become a key tool in the management of type 2 diabetes mellitus (T2DM). SFP‐1, an α‐glucosidase inhibitor derived from Sargassum fusiforme (S. fusiforme), was studied as a food‐derived hypoglycaemic agent. By binding α‐glucosidase and then changing the conformation of the enzyme, SFP‐1 with a mixed inhibition type exerted an excellent inhibitory effect (IC50 value: 0.681 mg mL−1). Structural analyses suggested that the average molecular weight of SFP‐1 was 1.61 × 105 Da, mainly composed of glucuronic acid, fucose, galactose, glucose, mannose and xylose, and the relative molar ratio was 23.07: 15.17: 7.72: 4.92: 4.04: 1.99. FTIR and NMR analysis suggested that SFP‐1 was an acidic polysaccharide with both α‐type and β‐type glycosidic bonds. S. fusiforme polysaccharide (SFP‐1) has the potential to develop as functional foods and nutraceuticals in T2DM management.