NpSb3 crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Np3+ is bonded to twelve equivalent Sb1- atoms to form a mixture of face and corner-sharing NpSb12 cuboctahedra. There are six shorter (3.23 Å) and six longer (3.43 Å) Np–Sb bond lengths. Sb1- is bonded in a 4-coordinate geometry to four equivalent Np3+ atoms.