(ZrO₂)_0.9(Sc₂O₃)_0.1 (10ScSZ) is an important commercial material for intermediate-to-high temperature solid oxide fuel cells. However, most 10ScSZ powders currently prepared by the solid-state reaction method suffer from large crystallite sizes and poor particle size uniformity; therefore, developing a simple, low-cost synthesis method with controllable particle and crystallite sizes remains crucial. This paper proposes an improved homogeneous co-precipitation method. By maintaining a stable pH value in the co-precipitation environment, 10ScSZ powder with controllable grain size and particle size was prepared. The three-step transformation process of the precursor—removal of water of crystallization, combustion decomposition of organic matter, and solid-state transformation—is also investigated. The crystallite size of the as-prepared 10ScSZ powders can be adjusted over a wide range of 10.5~31.9 nm by varying the pH of the co-precipitation environment and the calcination temperature. The primary particle size is 0.40~0.70 μm, and the powders exhibit a pure cubic phase. This synthesis route can also be extended to other rare-earth doped zirconia systems.