The Cs5BiP4Se12 salt grows naturally as nanowires that crystallize in the polar space group Pmc2(1), with a = 7.5357(2) angstrom, b = 13.7783(6) angstrom, c = 28.0807(8) angstrom, and Z= 4 at 293(2) K. The compound features octahedral [Bi(P2Se6)(2)](5-) coordination complexes that stack via weak intermolecular Se center dot center dot center dot Se interactions to form long, flexible fibers and nanowires. The Cs5BiP4Se12 fibers are transparent in the near- and mid-IR ranges and were found to exhibit a nonlinear optical second harmonic generation response at 1 urn that is approximately twice that of the benchmark material AgGaSe2. The material has a nearly direct band gap of 1.85 eV and melts congruently at 590 degrees C. Ab initio electronic structure calculations performed with the full-potential linearized augmented plane wave (FLAPW) method show that the band gap increases from its local density approximation (LDA) spin-orbit coupling value of 1.15 eV to the higher value of 2.0 eV when the screened-exchange LDA method is invoked and explain how the long nanowire nature of Cs5BiP4Se12 emerges.