Developing high‐performance gas sensors for room‐temperature (RT) operation remains a significant challenge in the field of gas detection. This study addresses this issue by designing and synthesizing a series of rare‐earth (RE) oxide modified ZnO heterojunctions (RE ₂ O ₃ /ZnO, RE = Nd, Y, Yb) via a co‐precipitation method. Systematic characterization combined with density functional theory (DFT) calculations reveals that the Yb ₂ O ₃ /ZnO heterostructure exhibits optimal morphology, abundant oxygen vacancies, and favorable band alignment. The resultant sensor demonstrates exceptional performance for trimethylamine (TMA) detection at RT, featuring a high response (6.84k% to 500 ppm), ultra‐fast response/recovery kinetics (8/14 s), excellent selectivity, and a low theoretical detection limit (0.926 ppm). The enhanced sensing mechanism is attributed to the strong Lewis acidity and high polarizability of Yb ³⁺ , which optimizes charge transfer and promotes TMA adsorption. Furthermore, the practical utility of the Yb ₂ O ₃ /ZnO sensor is successfully demonstrated through real‐time monitoring of fish spoilage, showing a strong linear correlation with storage time. This work not only presents a superior RT gas sensor but also provides deep insights into the role of RE cations in modulating heterojunction properties, offering a valuable strategy for designing advanced sensing materials.