Diabetes mellitus is a multifunctional disorder characterized by hyperglycemia resulting from increased hepatic glucose production, diminished insulin secretion resulting in impaired insulin action. The intestinal digestive enzymes α-glucosidase and α-amylase plays a key role in carbohydrate digestion, one main antidiabetic approach is to reduce the post prandial glucose level in blood by inhibition of alpha glucosidase and alpha amylase enzymes. Silver nanoparticles were prepared by green synthesis, where silver nitrate was taken as a metal precursor and marine red alga Halymenia poryphyroides as a reducing and capping agent. The formation of silver nanoparticles was characterized by UV–Nano photometer, FT-IR, SEM and XRD. In the present study invitro antidiabetic activity was studied from the biosynthesis of silver nanoparticles from the marine red alga Halymenia poryphyroides as a pre-requisite for the in vivo studies further. The assay results of silver nanoparticles showed dose dependent significantly (P<0.005) increase in percentage inhibitory activity against α–amylase enzyme, at a concentration of 0.2 mg/ml 26.20 ± 0.02% inhibition was seen and at 1.0 mg/ml 91.30 ± 0.02% inhibition was observed, similarly dose dependent significantly (P<0.005) increase in percentage inhibitory activity against α-glucosidase enzyme was also observed where in at lower concentration of 0.2 mg/ml 33.20 ± 0.01% of inhibition and at higher concentration of 1.0 mg/ml 89.10 ± 0.01% inhibition were recorded.