The nonstoichiometric ternary barium hafnium arsenide Ba0.8Hf12As17.7 has been synthesized through reaction of the elements at 950 degrees C in a tin flux, and its structure has been determined by single-crystal X-ray diffraction methods. It crystallizes in the hexagonal space group C-6h(2)-P6(3)/m with a = 14.997(1) Angstrom, c = 3.5526(4) Angstrom, and Z = 1 at 22 degrees C. The structure consists of columns of As-centered Hf-6 trigonal prisms aligned along [001]. It is derived from the Cr12P7 structure type (the anti-type of Th7S12) by inserting six-atom wide As ribbons between assemblies of the trigonal prismatic columns. Alternatively, Ba0.8HP12As17.7 may be considered to be an antitype of the Ho6Ni20P13 structure type but with nonmetal homoatomic bonding being a predominant feature. The weakly metallic behavior and some of the features of As-As bonding in Ba0.8Hf12As17.7 are supported by extended Huckel band structure calculations.