Herein,　a　simple　and　portable　electrochemical　sensor　based　on　a　reduced　graphene　oxide/gold　nanoparticle/manganese　dioxide　(rGO/AuNP/MnO2)　nanocomposite-modified　screen-printed　carbon　electrode　(SPCE)　was　constructed　by　the　facile　stepwise　electrodeposition　method　and　used　for　electrochemical　detection　of　As(III).　The　resultant　electrode　was　characterized　for　its　morphological,　structural,　and　electrochemical　properties　using　scanning　electron　microscopy　(SEM),　X-ray　photoelectron　spectroscopy　(XPS),　energy　dispersive　X-ray　spectroscopy　(EDX),　cyclic　voltammetry　(CV),　and　electrochemical　impedance　spectroscopy　(EIS).　From　the　morphologic　structure,　it　can　be　clearly　observed　that　the　AuNPs　and　MnO2　alone　or　their　hybrid　were　densely　deposited　or　entrapped　in　thin　rGO　sheets　on　the　porous　carbon　surface,　which　may　favor　the　electro-adsorption　of　As(III)　on　the　modified　SPCE.　It　is　interesting　that　the　nanohybrid　modification　endows　the　electrode　with　a　significant　decrease　in　charge　transfer　resistance　and　an　increase　in　electroactive　specific　surface　area,　which　dramatically　increases　the　electro-oxidation　current　of　As(III).　This　improved　sensing　ability　was　ascribed　to　the　synergistic　effect　of　gold　nanoparticles　with　excellent　electrocatalytic　property　and　reduced　graphene　oxide　with　good　electrical　conductivity,　as　well　as　the　involvement　of　manganese　dioxide　with　a　strong　adsorption　property　in　the　electrochemical　reduction　of　As(III).　Under　optimized　conditions,　the　sensor　can　detect　As(III)　via　square　wave　anodic　stripping　voltammetry　(SWASV)　with　a　low　limit　of　detection　of　2.4　μg　L−1　and　a　linear　range　of　25–200　μg　L−1.　The　proposed　portable　sensor　shows　the　advantages　of　a　simple　preparation　procedure,　low　cost,　good　repeatability,　and　long-term　stability.　The　feasibility　of　rGO/AuNPs/MnO2/SPCE　for　detecting　As(III)　in　real　water　was　further　verified.　©　2023　by　the　authors.