The small hepatitis B surface antigen (SHBs) is the most abundant hepatitis B virus (HBV) protein in individuals infected with HBV, and clearance of HBV surface antigen, which is primarily composed of SHBs, is considered a surrogate biomarker for achieving a functional cure of chronic HBV. Understanding SHBs degradation is crucial for its elimination and targeted eradication strategies. This study demonstrates that SHBs undergoes degradation via a ubiquitin/proteasome pathway, primarily through K48-linked ubiquitination, with K122 as the critical ubiquitination site. Utilizing immunoprecipitation and mass spectrometry, we identified TRIM21 (an E3 ubiquitin ligase) and OTUD4 (a deubiquitinase) as key regulators of SHBs. We verified the direct interaction between SHBs and TRIM21's coiled-coil domain, as well as the N-terminal amino acids 1-180 of OTUD4, using coimmunoprecipitation and glutathione S-transferase (GST) pull-down assays in both