Functions as a ubiquitin E3 ligase and as an ISG15 E3 ligase (PubMed:16352599). Involved in innate immune defense against viruses by mediating ubiquitination of DDX58 and IFIH1 (PubMed:17392790, PubMed:30193849, PubMed:33849980). Mediates 'Lys-63'-linked polyubiquitination of the DDX58 N-terminal CARD-like region and may play a role in signal transduction that leads to the production of interferons in response to viral infection (PubMed:17392790, PubMed:23950712). Mediates 'Lys-63'-linked polyubiquitination of IFIH1 (PubMed:30193849). Promotes ISGylation of 14-3-3 sigma (SFN), an adapter protein implicated in the regulation of a large spectrum signaling pathway (PubMed:16352599, PubMed:17069755). Mediates estrogen action in various target organs (PubMed:22452784). Mediates the ubiquitination and subsequent proteasomal degradation of ZFHX3 (PubMed:22452784). Plays a role in promoting the restart of stalled replication forks via interaction with the KHDC3L-OOEP scaffold and subsequent ubiquitination of BLM, resulting in the recruitment and retainment of BLM at DNA replication forks (By similarity). S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. ATP + [ISG15] + [protein]-lysine = AMP + diphosphate + [protein]-N-ISGyllysine. Protein modification; protein ubiquitination. Interacts (via SPRY domain) with DDX58 (via CARD domain). Interacts with ZFHX3. Interacts with NLRP12; this interaction reduces the E3 ubiquitin ligase TRIM25-mediated 'Lys-63'-linked DDX58 activation. Interacts with the KHDC3L/FILIA-OOEP/FLOPED scaffold complex and BLM at DNA replication forks (By similarity). Interacts with RTN3; this interaction prevents DDX58 ubiquitination (PubMed:34313226). (Microbial infection) Interacts (via coiled coil) with influenza A virus NS1 protein; this interaction specifically inhibits TRIM25 multimerization and TRIM25-mediated DDX58 CARD ubiquitination, thereby suppressing DDX58 signal transduction. (Microbial infection) Interacts (via SPRY domain) with human respiratory syncytial virus (HRSV) non-structural protein 1; this interaction suppresses DDX58 ubiquitination and results in decreased interaction between DDX58 and MAVS. (Microbial infection) Interacts with JC virus small t antigen; this interaction suppresses DDX58 ubiquitination thereby suppressing DDX58 signal transduction. (Microbial infection) Interacts with Severe fever with thrombocytopenia virus (SFTSV) NSs; this interaction this interaction sequesters TRIM25 in NSs-induced cytoplasmic inclusion bodies thereby inhibiting the IFN responses. Expressed in breast tumors (at protein level). Ubiquitous. By interferons. The RING-type zinc finger is important for ISG15 E3 ligase activity and autoISGylation. AutoISGylation negatively regulates ISG15 E3 ligase activity. The C-terminal B30.2/SPRY domain interacts with the first N-terminal CARD domain of DDX58. Auto-ISGylated. TRIM25 entry