Zn1−xNixO dense ceramics were prepared from Zn1−xNixO nanoparticles with x varying from 0 to 0.06. These nanoparticles were synthesized by liquid route. In the sintered samples, the solubility limit of Ni in the Zn1−xNixO wurtzite structure was found to be 0.03. The increase of x until 0.03 led to a significant raise in both electrical conductivity (σ) and absolute value of Seebeck coefficient (|S|). Ni-richer samples (x > 0.03) contained in addition a small amount of Ni rich secondary phase (ZnyNizO) with a cubic structure similar to NiO. The thermoelectric properties of all samples were investigated from room temperature to 1000 K. All doped samples showed a n-type semiconducting conductivity. For Ni contents higher than x = 0.03, the increase of the secondary phase content induced a decrease in σ and |S|. The highest power factor (0.6 mW m−1 K−2) and ZT (0.09) were found for Zn0.97Ni0.03O at 1000 K.