Using electrical conductivity measurements, we found the existence of different activation energy values for the electric transport in Ce1/3NbO3: 0.78 eV for T > 800 °C and 0.39 eV for T < 800 °C. Atomistic simulations have shown that the energy required to move the Ce3+ ions is around 8 eV, which is one order of magnitude higher of what is experimentally found. Additionally, electrical measurements at different partial pressures of oxygen show that the material has oxygen-ion conductivity, but the activation energy of 0.39 eV suggests other possible mechanisms of electrical conductivity. One of these possibilities is electronic transport, where the activation energy could be due to the band gap. To determine whether electronic conductivity is contributing in the low temperature regime, we performed ab-initio DFT electronic calculations to evaluate the gap, using the modified Becke–Johnson potential. Due to a Ce:4f level found in the gap, which obstructed the convergence of the calculation, we used the LDA + U approach on the Ce atoms, this moves the 4f levels out of the gap. We used the values U = 1, 2 and 3 eV, then extrapolated back to U = 0 to find the location of the Ce:4f state in the gap. The computed value of the activation energy for electronic conductivity is higher than the experimental one, and resembles more the optical gap value found in Ce1/3NbO3. Based on this result, it can be inferred that the electrical conductivity in Ce1/3NbO3 proceeds via anionic charge carriers in the entire temperature studied range.