Reversible protonic ceramic electrochemical cells (R‐PCECs) are emerging as highly efficient energy conversion devices, operating below 650 °C. The primary challenge in advancing R‐PCECs lies in developing efficient and stable proton‐conducting electrolytes capable of withstanding the chemical instability caused by common contaminants like CO₂ and H₂O. Herein, a novel high‐entropy perovskite oxide (HEPO) material is introduced, incorporating six equimolar B‐site cations (BaHf_1/6Sn_1/6Zr_1/6Ce_1/6Y_1/6Yb_1/6O_3‐δ, BHSZCYYb). The total conductivity of BHSZCYYb outperforms that of the examined HEPO electrolytes and other reported HEPO variants. Additionally, superior chemical stability of BHSZCYYb is observed when exposed to CO₂. Utilizing the microwave‐assisted sintering method, an R‐PCEC with BHSZCYYb electrolyte is successfully fabricated. This cell exhibits a maximum power density of 1.151 W cm⁻² (650 °C) in fuel cell mode and a current density of 2.326 A cm⁻² at 1.3 V (650 °C) in electrolysis cell mode. These results represent the highest‐reported values for R‐PCECs employing HEPO electrolytes to date and provide valuable insights into the development and advancement of HEPOs, holding great promise for achieving high‐performance R‐PCECs.