Niobium oxides have recently been demonstrated as excellent candidates to make memristors and memdiodes in neuristor circuits for application in neuromorphic computing. The Poole-Frenkel conduction in the niobium oxide layer is very sensitive to localized temperature and Joule heating. Additionally, thermal confinement and overheating in eventual microelectronics devices may result in significant degradation of performance and reliability. Therefore, it’s of great importance to understand and quantify thermal transport in these oxides. However, very few papers about thermal properties of niobium oxides have been published. Here, we report the first thermal conductivity (k) measurement of amorphous niobium oxide (a-Nb2O5-d) thin films by Time-domain Thermoreflectance (TDTR) method. We observe very low k of a-Nb2O5-d thin films (around 1 W/m-K) that are tunable (about 80% change) through varying oxygen vacancy concentrations (d). Additionally, the thickness dependence of k in a-Nb2O5-d films is studied to explore the influence of size effects in the context of locons, diffusons, and propagons. To complement this discussion, longitudinal wave velocities and vibrational energy spectra are measured by the picosecond acoustic method and Fourier-transform infrared (FTIR) spectroscopy, respectively.