β-Zn4Sb3 is a cheap nontoxic high-performance thermoelectric material, which unfortunately suffers from stability issues because of zinc migration in thermal and electrical gradients. Here, the thermoelectric properties and thermal stability of β-Zn4Sb3 mixed with varying sizes and weight percentages of TiO2 nanoparticles are investigated. Furthermore, the stability of pressed β-Zn4Sb3-TiO2 nanocomposite pellets is investigated by measuring high-energy synchrotron powder X-ray diffraction (PXRD) data during operating conditions using the Aarhus thermoelectric operando setup (ATOS). Through these studies, it is determined that TiO2 nanoparticle addition in pressed pellets of β-Zn4Sb3 does not prevent Zn migration, and even though effects are seen in the thermal conductivity and electrical resistivity, the overall zT remains unchanged regardless of TiO2 nanoinclusions. For the present samples, the Seebeck coefficients are unaffected by the addition of nanoparticles, and thus, there is no observed energy-filtering effect. The operando PXRD data reveal that the TiO2 nanoinclusions lower the degradation rate by up to 75%, but all samples eventually decompose. This is corroborated by long-term stability tests performed using a thermal gradient. In conclusion, TiO2 nanoinclusions do not degrade the excellent thermoelectric properties of β-Zn4Sb3, but the stabilizing effect is not sufficient for establishing long-term operating stability.