A nanocomposite consisting of gold nanoparticles deposited on the side walls of functionalised multi-walled carbon nanotubes, Ox-MWCNT-Aunano, was prepared using a simple chemical reduction. The nanoparticles were well dispersed with a mean diameter of 7.5 nm and had a face-centred cubic structure and a gold loading between 2.0% and 2.6% by weight. These gold decorated nanotubes were cast onto a gold electrode to form a uniform and homogeneous sensor. Using cyclic voltammetry, the reduction of Cr(VI) was observed at a peak potential of 0.52 V versus SCE in an acidified H2SO4 solution, pH 2.0. A linear calibration curve with a sensitivity of 0.28 mA mM− 1 and a LOD of 7.2 × 10− 7 M was obtained using constant potential amperometry coupled with rotating disc voltammetry. The electrochemical detection of Cr(VI) was also observed at a MWCNT-modified gold substrate but with a higher LOD, illustrating the advantage of combining the gold nanoparticles with MWCNTs. The sensor showed good selectivity for the detection of Cr(VI) in the presence of Cu(II), chloride and nitrates and in a real water sample. This was attributed to the electropositive reduction potentials of Cr(VI), the acidic H2SO4 supporting electrolyte that provides a well-known cleaning effect at gold, and the size and good dispersion of the gold nanoparticles that minimise particle agglomeration.