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Infectious bursal disease virus (IBDV), a double-stranded RNA virus, causes immunosuppression and high mortality in 3–6-week-old chickens. Innate immune defense is a physical barrier to restrict viral replication. After viral infection, the host shows crucial defense responses, such as stimulation of antiviral effectors to restrict viral replication. Here, we conducted RNA-seq in avian cells infected by IBDV and identified TRIM25 as a host restriction factor. Specifically, TRIM25 deficiency dramatically increased viral yields, whereas overexpression of TRIM25 significantly inhibited IBDV replication. Immunoprecipitation assays indicated that TRIM25 only interacted with VP3 among all viral proteins, mediating its K27-linked polyubiquitination and subsequent proteasomal degradation. Moreover, the Lys854 residue of VP3 was identified as the key target site for the ubiquitination catalyzed by TRIM25. The ubiquitination site destroyed enhanced the replication ability of IBDV in vitro and in vivo. These findings demonstrated that TRIM25 inhibited IBDV replication by specifically ubiquitinating and degrading the structural protein VP3. Author summary: Owing to the immunosuppression and high mortality caused by infectious bursal disease virus (IBDV) in chickens, hosts urgently boost their antiviral immune responses to resist IBDV infection. Here, we identified the host antiviral factor TRIM25, which defended against IBDV infection by direct targeting the viral protein VP3 for subsequent ubiquitination and degradation. To our knowledge, this is the first report of host TRIM25 restricting IBDV replication. Additionally, we found that Lys854 of VP3 was the target site ubiquitinated by TRIM25. Mutation in the ubiquitination site of VP3 abrogated the degradation of VP3 and further enhanced IBDV replication, but did not influence its virulence. Our work not only uncovers an all-important molecular mechanism of chicken TRIM25 against IBDV but also provides new insights for developing high-titer live IBDV vaccines. [ABSTRACT FROM AUTHOR]