Abstract One MeV negative particle beam accelerated in the beam source of the ITER Neutral Beam Injectors (NBIs) will be neutralised in the Neutraliser gas cell. Four narrow beam channels are foreseen in the Neutraliser where the neutralisation process will occur with controlled gas pressure being the four channels delimited by five copper wall panels. Stray particles will be dumped on the copper Electron Dump and CuCrZr leading edges to be installed at the Neutraliser frontal section: the Electron Dump will intercept stray electrons in order to reduce the cryo pump thermal load; enhanced heat transfer in subcooled boiling conditions will occur in the panel leading edges with twisted tapes as turbulence promoters. The copper panels will be thermally controlled by means of embedded cooling circuits; thermo-hydraulic and thermo-mechanical analyses and verifications have been carried out considering several load combinations and satisfying the design rules as for ITER structural design criteria for in vessel components. Gas flow analyses have been carried out with molecular flow in the in-vessel vacuum environment to evaluate the gas pressure profile along the beam line also considering the presence of the Electron Dump. Furthermore, transient analyses of the gas flow inside channels have been performed to simulate the effect of last valve closure; analysis results demonstrate that gas flow variations can be detected by thermal measurements. The Neutraliser assembly, installation, and positioning inside the vacuum vessel have been verified considering alignment requirements and remote handling compatibility with twistlock crane, bolting tool, and pipe joint interfaces. The Neutraliser and Electron Dump will be instrumented with thermocouples foreseen for component protection, thermal control, beam diagnostic, and calorimetry. Additional thermocouples, accelerometers as boiling detectors, and further diagnostics for component monitoring and beam characterisation are also foreseen in the NBI prototype located in Padua and named MITICA. Qualification and factory tests have been determined according to the required standards and on-site acceptance tests have been preliminarily identified. Some technological issues, common to other beam line components of ITER NBIs, have been investigated by carrying out R&D activities which are also presented in the paper.