We report on a detailed investigation of the crystal and electronic band structures and ofthe transport and thermodynamic properties of the Mo-based cluster compound Ag2Tl2Mo9Se11. Thisnovel structure type crystallizes in the trigonal space group R3̅c and is built of a three-dimensionalnetwork of interconnected Mo9Se11 units. Single-crystal X-ray diffraction indicates that the Ag and Tlatoms are distributed in the voids of the cluster framework, both of which show unusually largeanisotropic thermal ellipsoids indicative of strong local disorder. First-principles calculations show aweakly dispersive band structure around the Fermi level as well as a semiconducting ground state. Theformer feature naturally explains the presence of both hole-like and electron-like signals observed in Halleffect. Of particular interest is the very low thermal conductivity that remains quasi-constant between150 and 800 K at a value of approximately 0.6 W·m−1·K−1. The lattice thermal conductivity is close to itsminimum possible value, that is, in a regime where the phonon mean free path nears the meaninteratomic distance. Such extremely low values likely originate from the disorder induced by the Ag andTl atoms giving rise to strong anharmonicity of the lattice vibrations. The strongly limited ability of this compound to transportheat is the key feature that leads to a dimensionless thermoelectric figure of merit ZT of 0.6 at 800 K.