The crystal structure of a new niobium phosphate bronze, Nb18P2.5O50, was determined by single-crystal X-ray diffraction, and its physical properties were studied. Nb18P2.5O50 crystallizes in the tetragonal system with space group I4/m (No. 87) and Z = 1. The unit cell parameters are a = 15.593(1) Angstrom, c = 3.8282(3) Angstrom, and V = 930.8(1) Angstrom(3). The structural refinement by a fun-matrix least-squares technique led to R = 0.055 (R(w) = 0.052). The structure of Nb18P2.5O50 is built UP from 3 X 3 slabs of ReO3-type NbO6 octahedra. These slabs are stacked alternatively along the crystallographic c axis and joined together by edge-sharing In the ab plane, the slabs of NbO6 octahedra are interconnected through corner-sharing PO4 tetrahedra. Single-crystal resistivity measurement of Nb18P2.5O50 along the crystallographic c axis showed a semiconducting behavior with an activation energy of 0.13(1) eV. The magnetic susceptibility data indicate a modified Curie-Weiss behavior with the Curie-Weiss temperature theta = -3.70 K and mu(eff)/Nb4+ = 0.8 mu(B).