NiO nanoparticles with an average size of 15 nm were prepared by a simple, reproducible and low-cost controlled method, using nickel nitrate hexahydrate (Ni(NO 3 ) 2 ⋅6H 2 O). These nanoparticles were added to an agarbased polymer electrolyte formula, resulting, after reflux and solution casting, in a proton conducting membrane. The highest ionic conductivity values of 5.19×10-5 S cm -1 at room temperature and 3.32x10 -4 S cm -1 at 80 °C, were obtained for the sample with 50 wt. % of acetic acid and 0.25 g of NiO. Moreover, the samples showed 75 % of transparency in the visible region, a homogeneous surface and mainly amorphous structure. All the obtained results suggest that agarbased polymer electrolyte with NiO nanoparticles are promising candidates for electrochemical devices application.