The structural chemistry of the complete binary neptunium–silicon system has been investigated by means of X-ray diffraction. This system is characterised by the existence of six different compounds, whereas only three were previously reported in the literature. The formation and the crystal structure of the NpSi3 compound (AuCu3 type) and of the richest in neptunium binary Np3Si2 compound (U3Si2 type) were confirmed. The ThSi2 type of compound was also confirmed but, as usually observed in the rare-earth or actinide systems, with a slight defect on the silicon site. The hexagonal AlB2 type compound was isolated for the first time, showing, as expected, a higher silicon defect than in the ThSi2 type of compounds. The last phase observed in this system was found to crystallise with the NpSi composition showing a polymorphic transformation. Depending on the annealing process a primitive orthorhombic FeB type, as observed with the rare-earth systems, or a C-centered orthorhombic new crystal structure type are obtained. These new results are described and compared with the recently revised Np–Ge and uranium homologue systems.