SUMMARYClubroot is a major threat to cruciferous crops worldwide, largely due to the complex pathogenesis of its causal agent, Plasmodiophora brassicae, and the limited availability of genetic resistance in plants. Previous research has shown that P. brassicae secretes the E3 ubiquitin ligase PbE3‐2, which targets and degrades the Arabidopsis thaliana cysteine protease RD21A to facilitate infection. In this study, we identified a plant defense mechanism that counteracts this pathogen virulence strategy. We found that the A. thaliana deubiquitinating enzyme OTU2, whose expression is upregulated during infection, interacts with RD21A. Notably, OTU2 stabilized RD21A by deubiquitination and inhibited the interaction between PbE3‐2 and RD21A. Furthermore, OTU2 overexpression enhanced A. thaliana resistance to P. brassicae in an RD21A‐dependent manner. Collectively, our findings demonstrate that OTU2 deubiquitinates RD21A, protecting it from PbE3‐2‐mediated degradation and thereby mitigating P. brassicae virulence. This study provides new insights into plant immune mechanisms and offers potential strategies for developing clubroot‐resistant crops.