Herein, the novel synthesis, characterisation and further application of magnetic multi-walled carbon nanotubes modified with polyamidoamine (PAMAM) dendrimers (MMWCNTs-D-NH2) as highly sensitive and selective recognition element in sensor development for the electrochemical determination of As(III) in water matrices is presented. The synthetized nanomaterials were characterised by Fourier transform- infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. Thus, the glassy carbon electrode (GCE) was subjected to functionalisation with the MMWCNTs-D-NH2 to produce the sensor. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetric (CV) measurements were performed to characterise the surface of modified electrode. The electrochemical performances of the developed sensor towards different concentration of As(III) were studied by square wave anodic stripping voltammetric (SWASV) measurements. The sensor shown its high sensitivity and selectivity against possible interferents, such as Hg(II), Pb(II), Cd(II), Cu(II). The limit of detection (LOD) was found to be 0.46 µgL−1. The suitability of the proposed sensor in spiked water samples (drinking, tap and mineral water) is also discussed.