Two new quaternary polyselenides, Ba4TMSbSe12 (TM = Nb, Ta), are synthesized using a solid state reaction. They crystallize in a new structural type with a P-1 triclinic space group, characterized according to single-crystal X-ray diffraction. The structure is alternately stacked using isolated NbSe95− and SbSe33−, which are separated by Ba2+. The structure contains the NbSe95− unit with a bipentagonal pyramidal shape coordinated with Se2− and Se22− in monodenteate and bidentate modes. The vibrational property of the diselenide Se22− unit was studied using Raman spectrum analysis. ultraviolet–visible diffused reflectance and temperature-dependent resistivity measurements indicate semiconductor behaviors. Calculations of electronic structures indicate the presence of a band gap and strong Se-Se interactions in the diselenide group, which experimentally supports the measured physical properties.