One of the modes of diabetes treatment is the inhibition of diabetes inhibitor enzymes like α-amylase and α-glucosidase. The search for natural drugs with fewer side effects has led to the identification of certain species of seaweeds showing inhibition of these enzymes. This study deals with the effectiveness of ethanolic extract of the brown seaweed Sargassum wightii and its polysaccharide; fucoidan on α-amylase and α-glucosidase using in vitro study. Materials and methods: Crude extract and fucoidan were prepared from S.Wightii. The standard drug used for comparison was acarbose. The inhibitory effect of the seaweed derivatives on α-amylase and α-glucosidase were analysed. Results: IC50 values for α-amylase inhibitory activity were 394.2 µg/mL for S.wightii ethanolic extract, 222.6 µg/mL for fucoidan, and 96.6 µg/mL for acarbose. Statistical analysis by two-way ANOVA indicated that the variation in inhibitory activity due to concentration was statistically significant (P < 0.001) for all the tested samples including acarbose. Regarding the α-glucosidase enzyme, irrespective of the tested samples, a concentration-dependent inhibitory activity was noted. The IC50 values for α-glucosidase inhibitory activity were 320.7 µg/mL for S.wightii ethanolic extract, 196.0 µg/mL for fucoidan, and 42.2 µg/mL for acarbose, respectively. Two-way ANOVA indicated that the variation in α-glucosidase activity due to the concentration of tested samples and acarbose was statistically significant (P < 0.001). Conclusion: The inhibitory effect on α-amylase and α-glucosidase by the isolated molecule; fucoidan from S. wightii was higher compared to the crude ethanolic extract. Hence, fucoidan from the seaweed S. wightii can be used as an adjunct for the treatment of diabetes.