Abstract Background: The immunotherapy for different types of cancers that targeting programmed death protein-1 (PD-1) and programmed death ligand-1 (PD-L1) has highlighted the importance of suppressing specific T cell responses. Recently, several studies have shown that the expression level of PD-L1 in tumor cells is positively correlated with tumor metastasis as well as recurrence rate. The potent effects of post-translational modifications (PTMs) for PD-L1, such as ubiquitination, glycosylation, phosphorylation and palmitoylation, have been reported to be related to immunosuppression. However, the regulation of PD-L1 degradation in cancers is still not well understood. In this paper, we mainly investigate the deubiquitination regulation of PD-L1. Methods: The protein levels of PD-L1 and USP21 were detected by Immunoblotting and immunohistochemistry. The interaction between PD-L1 and USP21 was determined by co-immunoprecipitation. The deubiquitination of PD-L1 was determined by in vitro deubiquitination assay. The deubiquitination sites of PD-L1 were identified by mass spectrometry analysis. The expression of mRNA in target tissues was presented by bioinformatics analysis.Results: Overexpression of USP21 significantly increased PD-L1 abundance and knockdown of USP21 induced degradation of PD-L1. In vitro deubiquitination assay showed that USP21-WT reduced polyubiquitin chains from PD-L1 while USP21-C221A did not. Furthermore, five lysines in intracellular segment of PD-L1 are potential deubiquitin sites and cancer-derived mutations of PD-L1 in Asp276 have the ability to enhance the deubiquitination of PD-L1 mediated by USP21. Finally, we found that USP21 is the frequently amplified deubiquitinase in lung cancer, especially in lung squamous cell carcinoma, and its amplification co-occurs with the upregulation of PD-L1 levels. Moreover, IHC analysis showed stronger staining of PD-L1 and USP21 in lung cancer samples than adjacent tissues. Conclusion: We identified USP21 as a novel deubiquitinase of PD-L1. Hopefully, targeting PD-L1 by inhibiting USP21 might be a potentially novel strategy for the treatment of lung cancer.