The ternary transition-metal arsenide Nb9PdAs7 has been prepared through reaction of the elements, and its structure has been determined by single-crystal X-ray diffraction methods. It adopts a new structure type (Pearson symbol hP51, hexagonal, space group P (6) over bar, Z = 3), with unit cell parameters a = 16.6955(6) and c = 3.5582(l) Angstrom. The structure contains assemblies of As-centered trigonal prisms that extend as triangular columns through sharing of the triangular faces. Not only does Nb9PdAs7 extend a family of hexagonal structures with general formula (Mn+3n+2Xn+nY)-X-2-Y-2 to n = 4, the highest member known thus far, but it also displays the unique feature in which there are two distinct types of triangular columns, one having corner atoms (Pd) different from the other atoms (Nb). Structural relationships between members of the (Mn+3n+2Xn+nY)-X-2-Y-2 family are presented. The chemical bonding in Nb9PdAs7 was analyzed through an extended Huckel band structure calculation.