Samples with the nominal composition Cu0.9Pb1.2Sb2.9Se6 mainly contain a phase with incommensurately modulated lillianite-type structure with the respective composition. Single crystal diffraction with synchrotron radiation enabled a detailed refinement using the superspace group Cmcm(α00)00s with lattice parameters a = 4.16537(5), b = 14.0821(2), c = 19.8234(3) Å, and a modulation vector q = 0.6890(2)a* at room temperature. The structure is built up from tilted and distorted NaCl-type slabs that are interconnected by bicapped trigonal prisms, which mainly host Pb atoms, according to a 4L arrangement. Satellites up to the second order reveal positional and occupational modulation that mainly involves a sequence of Sb and Cu atoms and allows the Se substructure to adapt in a way that Sb and Cu feature predominantly octahedral and tetrahedral coordination, respectively. Above 523 K, satellite reflections disappear, and the crystal structure becomes more disordered with average coordination spheres of both Sb and Cu atoms corresponding to distorted octahedra. This phase transition leads to discontinuities in the evolution of lattice parameters and physical properties as functions of temperature. HRTEM investigations corroborate centrosymmetry and highlight atoms that are strongly affected by the modulation. Measurements of transport properties reveal a p-type semiconductor with a thermoelectric figure of merit up to 0.1 at 623 K. In accordance with B factor analysis, a small amount of substitution could increase zT significantly by optimizing the carrier concentration.