Phase pure samples of Nb21S8 are obtained by chemical transport and molten flux reactions in sealed niobium containers at 850-950 degrees C. In the temperature range from 5 to 290 K the electrical conductivity is found to be moderate metallic with a specific resistivity of 3.90 m Omega cm at 273 K. Magnetic susceptibility measurements give weak, almost temperature independent paramagnetism above similar to 40 K. These metallic properties are compared with the structure of Nb21S8, which contains linear single and double chains of fused body centered niobium cubes, separated by S and additional Nb atoms. Both physical measurements consistently show a transition into the superconducting state below 4.1(5) and 3.7(2) K, respectively. Recently ideas were developed that superconductivity is favored by the tendency of pairwise attraction of conducting electrons at the Fermi level. To see whether these ideas can be applied to Nb21S8, electronic band structure calculations (TB-LMTO-ASA-method) have been performed. In fact at the X symmetry point the band structure shows two saddle points near the Fermi level giving rise to electron pairing. Other bands cut the Fermi level with large dispersion. These two band structure features are discussed as chemical requirements (fingerprints) for the occurrence of superconductivity in this three-dimensionally tightly bonded phase. Thereby, the applicability of this chemical picture of superconductivity is demonstrated on this system.