The boric acid flux reaction of NpO2(ClO4)(2) with NaClO4 affords Na[(NpO2)(4)B15O24(OH)(5)(H2O)](ClO4)center dot 0.75H(2)O (NaNp-BO-1). NaNpBO-1 possesses a layered structure consisting of double neptunyl(VI) borate sheets bridged by another Np-VI site through cation cation interactions. The sole presence of Np-VI in NaNpBO-1 is supported by absorption and vibrational spectroscopy.