Dementia is a chronic progressive mental disorder, which adversely affects memory, thinking, comprehension, calculation and language. Some of the commonest forms of dementia are Alzheimer's disease, Parkinsonism, Dementia with Lewy Bodies and Myasthenia gravis. All these disorders are related to abnormalities in the central cholinergic system, which shows a decline in acetylcholine level. Cholinesterase (ChE) inhibitors are one of the novel strategies used for the symptomatic treatment of neurological disorders like dementia. In the course of screening new ChE inhibitors from marine sources, about 11 seaweeds, which have wide pharmaceutical applications, were collected from Hare Island, Gulf of Mannar, Tamilnadu, India. Methanolic extracts of the seaweeds were assessed for ChE inhibitory activity under in vitro conditions. Kinetic parameters IC(50), K(i) and V(max) were also analysed. The results showed that 3/11 seaweeds showed 50% inhibition for both ChEs (using acetylthiocholine iodide and butyrylthiocholine iodide as substrate) at concentrations of 2 mg mL(-1) (Gracilaria gracilis, Sargassum, Cladophora fasicularis for ChE with acetylthiocholine iodide as substrate and Gracilaria gracilis, Gracilaria edulis, Sargassum for ChE with butyrylthiocholine iodide as substrate) and 4/11 showed no inhibitory activity. Inhibitory activity of seaweed extracts was compared with standard drug donepezil. Enzyme kinetic analysis showed that algal extracts exhibited mixed type inhibition (partially non-competitive inhibition).