Oxygen-ionic and proton-conducting oxides are widely studied materials for their application in various electrochemical devices such as solid oxide fuel cells and electrolyzers. Rare earth oxides are known as a class of ionic conductors. In this paper, La2ZnNdO5.5 and its Ca-doped derivatives La2Nd0.9Ca0.1ZnO5.45 and La2ZnNd0.9Ca0.1O5.45 were obtained by a solid-state reaction route. Phase composition, lattice parameters, and hydration capability were investigated by X-ray diffraction and thermogravimetric analyses. The conductivities of these materials were measured by the electrochemical impedance spectroscopy technique in dry (pH2O = 3.5 × 10−5 atm) and wet (pH2O = 2 × 10−2 atm) air. All phases crystallized in a trigonal symmetry with P3m1 space group. The conductivity difference between undoped and calcium-doped samples is more than two orders of magnitude due to the appearance of oxygen vacancies during acceptor doping, which are responsible for a higher ionic conductivity. The La2Nd0.9Ca0.1ZnO5.45 sample shows the highest conductivity of about 10−3 S∙cm−1 at 650 °C. The Ca-doped phases are capable of reversible water uptake, confirming their proton-conducting nature.