The novel paper-based colorimetric device using the hydride generation and Au(III) was developed for the determination of As(III) using a smartphone. The analysis of As(III) is based on arsine gas from hydride generation oxidized Au(III) to Au(0). In the presence of As(III), the distinctive color change of the Au(III) from colorless to violet was visualized by the naked eyes within 3 min. The Auenhanced Au particles strategy as the seeding method was performed to improve the sensitivity of the developed device. For the enhancing step, Au(0) was enlarged by the catalytic reaction of K[AuCl₄] and ascorbic acid. Under optimal conditions, a good linear relationship between the logarithm of arsenic concentration and the average mean color intensity was found in the arsenic range of 5 - 1000 ppm. The limit of detection (LOD) was observed as low as 1 ppm (3SD/slope). Furthermore, the developed device presents high selectivity against common heavy metals interferences. The proposed device shows excellent benefits, for example, low cost, ease of use, less time requirement and high selectivity.