NpAl3 crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Np is bonded to twelve equivalent Al atoms to form a mixture of corner and face-sharing NpAl12 cuboctahedra. There are six shorter (3.04 Å) and six longer (3.09 Å) Np–Al bond lengths. Al is bonded in a 10-coordinate geometry to four equivalent Np and six equivalent Al atoms. There are two shorter (2.85 Å) and four longer (2.87 Å) Al–Al bond lengths.