Single crystals of six new non-stoichiometric quaternary chalcogenides with a general formula Ba2−δLn1−xMn2−yQ5 (Ln = Pr, Nd, and Gd; Q = S and Se) have been synthesized by the molten flux method. Single crystal X-ray diffraction (SCXRD) studies show that these compounds are isostructural and crystallize in the monoclinic space group C2/m with two formula units. These structures show occupational disorder at the metal sites, and the SCXRD refined compositions of these compounds are Ba2Pr0.88(1)Mn1.79(1)S5, Ba2Nd0.83(1)Mn1.89(1)S5, Ba2Gd0.82(1)Mn1.77(1)S5, Ba1.86(1)Pr0.61(1)Mn1.88(1)Se5, Ba1.90(1)Nd0.57(1)Mn1.93(1)Se5, and Ba1.88(1)Gd0.56(1)Mn1.95(1)Se5. The sulfide structures have split positions for Mn atoms with the concomitant occupational disorder at the Pr and Mn sites. In contrast, the selenide structures do not exhibit split Mn sites. The Ba sites in these selenides also show occupational disorder along with partially occupied Ln and Mn sites. The crystal structures of the selenides are comprised of [2∞Ln1−xMn2−ySe5]n− layers separated by Ba2+ cations. The Ln atoms in these sulfides and selenides are octahedrally coordinated with six Q atoms, and each Mn atom is bonded with four Q atoms forming a distorted tetrahedral geometry. The charge balancing in these structures is difficult due to the metal vacancies. The optical absorption study on the polycrystalline Ba1.88Gd0.56Mn1.95Se5 sample reveals a direct bandgap of 1.73(2) eV in agreement with the reddish color of the sample. The density functional calculations are performed to study the atomic and electronic structures of Ba1.88(1)Gd0.56(1)Mn1.95(1)Se5. © 2022 Elsevier B.V.