A systematic investigation of the ternary system Yb-Mn-Sb led to the discovery of the novel phase Yb10MnSb9. Its crystal structure was characterized by single-crystal X-ray diffraction and found to be complex and highly disordered. The average Yb10MnSb9 structure can be considered to represent a defect modification of the Ca10LiMgSb9 type and to crystallize in the tetragonal P42/mnm space group (No. 136) with four formula units per cell. The structural disorder can be associated with both occupational and positional effects on several Yb and Mn sites. Similar traits were observed for the structure of the recently reported Yb21Mn4Sb18 phase (monoclinic space group C2/c, No. 15), which was reevaluated as part of this study as well. In both structures, distorted Sb6 octahedra centered by Yb atoms and Sb4 tetrahedra centered by Mn atoms form disordered fragments, which appear as the hallmark of the structural chemistry in this system. Discussion along the lines of how difficult, and important, it is to distinguish Yb10MnSb9 from the compositionally similar binary Yb11Sb10 and ternary Yb14MnSb11 compounds is also presented. Preliminary transport measurements for polycrystalline Yb10MnSb9 indicate high values of the Seebeck coefficient, approaching 210 μV K-1 at 600 K, and a semiconducting behavior with a room-temperature resistivity of 114 mΩ cm.