Sesquioxide ceramics are promising candidates for high-power laser applications, yet achieving high optical quality remains challenging. This is primarily due to rapid grain-boundary mobility during the final stage of sintering and pump-induced photodarkening under high-power excitation, particularly when traditional tetravalent sintering additives (e.g., ZrO₂) are used. Here, approximately 1 : 1 (molar) Zr⁴⁺–Ca²⁺ codoping is introduced to regulate the sintering behaviors of Yb:Y₂O₃ ceramics, yielding dense microstructures with uniform elemental distributions. The charge compensation enabled by Zr–Ca codoping suppresses the formation of point defects, thereby significantly mitigating photodarkening. As a result, the optimized 0.02 at% Zr–Ca codoped sample delivers a maximum continuous-wave (CW) output power of 20.5 W at 1076 nm with a slope efficiency of 65.4%. These results demonstrate that Zr–Ca codoping is an effective strategy for fabricating high-performance sesquioxide laser ceramics with improved optical quality and enhanced resistance to pump-induced photodarkening.