The synthesis, characterization, crystal structure, thermal stability, optical spectroscopic, electronic structure, and second-order nonlinear optical (NLO) properties of a novel class of materials, based on planar [N4]Ni(II) complexes, are reported. The single-crystal X-ray structure of the dichloro derivative indicates a planar structure of the molecule. The Ni(II) ion is tetracoordinated in a square planar environment, with the four nitrogen atoms as donors. The unsubstituted derivative exhibits efficiency 1.2 times that of urea in second-harmonic generation at 1.9 μm. INDO/SCI-SOS quantum-chemical calculations predict a sizeable molecular response and a significant octupolar contribution to optical nonlinearity.