The present study was intended to standardize the protocol for the synthesis of silver nanoparticles (AgNPs) using aqueous extract of Dictyota bartayresiana J.V. Lamouroux and evaluate cytotoxic potentials using brine shrimp bio-assay activity and trypan blue dye exclusion method. The reduction of pure Ag+ ions was monitored by measuring the UV-Vis spectrum of the solution at 200-900 nm and the characteristic peaks were detected. FTIR analysis was used to detect the characteristic peaks and their functional groups. The powder method of diffraction was employed for structural characterisation of AgNPs . The cytotoxic and anticancer potentials were evaluated by, the brine shrimp bio-assay and trypan blue dye exclusion method against DLA cell lines, repectively . When the D. bartayresiana aqueous extract was mixed with 1 mM AgNO3 solution, the colour of the solution changed from pale yellow to yellowish brown colour. The AgNPs synthesized from aqueous extract of D. bartayresiana showed an absorption of 0.639 at 410 nm. The broadening of peaks indicated that the particles are polydispersed. The capping was confirmed by the existence of bands at 1019.19, 1642.09, 1643.05 and 3401.82 cm–1. The D. bartayresiana AgNPs illustrated nine peaks at 2θ values were 26.661°, 28.388°, 29.947°, 32.244°, 40.553°, 46.223°, 50.19°, 54.78°, 57.448° and 76.67° corresponding to 208, 509, 149, 1681, 257, 833, 178, 275, 272 and 256 planes of silver, respectively. The AgNPs of D. bartayresiana revealed 50% mortality (LC50) of brine shrimp nauplii at 196.5 µl/l. Concentration needed for 50% inhibition of growth of DLA cells was found to be 296.14 µl/l of D. bartayresiana AgNPs.  The results of the present study demonstrated a simple, rapid and economically cheap route to synthesize AgNPs using aqueous extract of Dictyota bartayresiana thallus. Cytotoxic studies against Artemia salina confirmed that AgNPs are capable of rendering high cytotoxic activity and hence has a great potential in the preparation of anti-cancer drugs. The synthesized AgNPs may improve the therapeutic and medicinal values of Dictyota bartayresiana.