Two edible seaweeds, Sargassum polycystum and Sargassum wightii, were investigated for their antidiabetic potential using in vitro enzyme inhibitory assays. Among the various extracts, petroleum ether and ethyl acetate extracts of S. wightii showed significant inhibitory effects against α-amylase (IC50 378.3 µg/ml) and α-glucosidase (IC50 314.8 µg/ml). Methanol extract of S. wightii showed the highest inhibition against dipeptidyl peptidase-IV (DPP-IV) (IC50 38.27 µg/ml) and moderate antioxidant activity was observed in acetone extract of S. wightii (44%). Similarly, ethyl acetate extract of S. polycystum showed the highest inhibition against α-amylase (IC50 438.5 µg/ml) and methanol extract of S. polycystum showed maximum inhibition against α-glucosidase (IC50 289.7 µg/ml) and DPP-IV (36.94 µg/ml). The antioxidant activity was poor (22%). The extracts were investigated for in vitro cytotoxicity, DNA fragmentation in macrophages and haemolytic activity against erythrocytes, but no notable toxicity was observed with any of the tested extracts. Gas chromatography–mass spectrometry revealed the presence of the antidiabetic compound fucosterol and other major bioactive compounds, giving an insight into the antidiabetic and antioxidant properties of these algae. This study reveals the possible mechanisms of antidiabetic action in vitro, and these two seaweeds may also have an antidiabetic action in vivo. © 2015, © 2015 Taylor & Francis.