A new eight‐layer hexagonal perovskite Ba8NiNb6O24 was synthesized via the high‐temperature solid‐state reaction and its structure was characterized using selected area electron diffraction, high‐resolution transmission electron microscopy, and synchrotron X‐ray diffraction. Unlike the eight‐layer ordered shifted Ba8CoNb6O24 and Ba8ZnNb6O24, Ba8NiNb6O24 adopts a twinned structure with stacking sequence (ccch)2 for the BaO3 layers and displays more disordered cation and vacancies over the face‐sharing octahedral (FSO) sites than the twinned tantalates Ba8MTa6O24 (M=Zn, Ni, Co). The stabilization of twinned structure and cation/vacancy ordering in Ba8NiNb6O24 composition is correlated with the smaller size difference between Ni2+ and Nb5+ in comparison with those between (Zn/Co)2+ and Nb5+ in the shifted Ba8CoNb6O24 and Ba8ZnNb6O24. The Ba8NiNb6O24 pellet exhibits high dielectric permittivity εr ~ 40, modest Qf ~ 41 319 GHz, and large temperature coefficient of resonant frequency τf ~ 60 ppm/°C. The lower Qf value compared with the high‐Q Ba8MTa6O24 is ascribed to the reduced short‐range B‐cationic ordering inside the FSO dimers in Ba8NiNb6O24. These results contribute to understanding the interplay among chemical composition, structure, and dielectric properties of the eight‐layer twinned and shifted hexagonal perovskites.