A pseudoternary phase diagram of the BaO-ZrO2-ScO1.5 system is established and the conductivities of the scandium-doped barium zirconate are measured in wet conditions. According to the phase diagram, the molar fraction of scandia in the barium zirconate phase that equilibrates with the cubic zirconia (ZrO2) and scandia (Sc2O3) phase is 0.03. On the composition line, where the molar fraction of barium oxide (BaO) is 0.50, the solubility of scandia into barium zirconate is 0.27 as a molar fraction of scandia. At around 500°C, the conductivities of scandium-doped barium zirconate are estimated to be almost same as that of yttrium-doped barium zirconate. The activation energies for bulk proton conduction in scandium-doped barium zirconate (0.57 eV for BaZr 0.90Sc0.10O3-δ) are higher than that of yttrium-doped barium zirconate (0.44 eV for BaZr0.90Y 0.10O3-δ). In addition, the bulk conductivity of scandium-doped barium zirconate in this study is 10 times higher than the reported conductivities in the temperature range of 75°C to 300°C.