The new ternary bismuth sulfide KBi3S5 forms from the reaction of Bi in a K2Sx flux at 300 degrees C (94% yield). The compound crystallizes in the orthorhombic space group Pmna (no. 62) with a = 17.013(5) Angstrom, b = 4.076(2) Angstrom, c = 17.365(4) Angstrom, V = 1204(2) Angstrom(3), and Z = 4 (final R/R(w) = 5.1/6.7%), data with F-0(2) > 3 sigma(F-0(2)) 782, no. of variables 59, 2 theta(max) = 50 degrees. The compound contains an open framework composed of edge-sharing BiS6 octahedra to form large tunnels which are occupied by K+ ions fractionally distributed over two sites. The S-S distances across the channel range from 9.45-14.02 Angstrom. KBi3S5 is a semiconductor with a band-gap of 1.21 eV. It undergoes a structural change upon heating at 520 degrees C. KBi3S5 exhibits topotactic ion-exchange with RbCl to give beta-RbBi3S5 at 350-380 degrees C. Ion-exchange with aqueous HCl in air forms an oxidized, metastable bismuth sulfide which undergoes endothermic decomposition to Bi2S3 at similar to 140 degrees C.