A simple and fast amperometric sensor for As(III) determination has been developed using alginic acid (AA) and nPEDOT on gold nanoparticle-modified screen-printed carbon electrodes (AuNP-SPCE). To obtain a selective method for As(III) determination, the effects of various parameters, such as pH, supporting electrolyte, AA and nPEDOT concentration (C-AA, C-nPEDOT), and applied potential (E-app), were studied. The optimal experimental conditions were chosen to be as follows: phosphoric acid pH 3.0 (0.05 mol L-1); C-AA: 0.50 mu g L-1 and C-nPEDOT 50% w/v (E-app: -0.30V). The peak current was proportional to the As concentration between 5.0 and 25.0 mu g L-1 (R=0.9903), with a detection limit (DL, 3 sigma) of 2.7 mu g L-1 and a sensitivity of 0.0481 mA/mu g L-1. Four different modified electrodes were used in this study: AuNPs-SPCE, AA/AuNPs-SPCE, nPEDOT/AuNPs-SPCE and AA/nPEDOT/AuNPs-SPCE. It was found that in the presence of AA and nPEDOT, the current of the arsenic signal was higher than that obtained using each of the compounds individually. AA allowed the adsorption of the arsenic on the electrode obtaining good sensitivity and nPEDOT, an excellent conductor, improved the electronic transfer in the electroanalytical system. The advantages of this new procedure are fast complexation kinetics, high specificity, simplicity and speed. Finally, the best electrode (AA/nPEDOT/AuNPs-SPCE) was applied to the determination of total arsenic in a water sample from the Loa River (North zone) and two samples from the Central zone (Santiago and Fifth region) with satisfactory results. Values obtained were compared with an ICP-MS method. To determine As-total, As(V) was reduced in the presence of thiosulfate in an acidic medium.