A new compound with one-dimensional spin chains, Ba9Co3Se15, was synthesized under high pressure and high temperature conditions and systematically characterized via structural, transport and magnetic measurements. Ba9Co3Se15 crystallizes in a hexagonal structure with the space group P-6c2 (No. 188) and lattice constants of a=b=9.6765 ? andc=18.9562 ?. The structure consists of trimeric face-sharing octahedral CoSe6 chains, which are arranged in a triangularlattice in the ab-plane and separated by Ba atoms. The distance of the nearest neighbor of CoSe6 chains is very large, givenby the lattice constant a=9.6765 ?. The Weiss temperature Tθ associated with the intra-chain coupling strength is about?346 K. However, no long-range magnetic order but a spin glass transition at～3 K has been observed. Our results indicate that the spin glass behavior in Ba9Co3Se15 mainly arises from the magnetic frustration due to the geometrically frustratedtriangular lattice.