Tripartite motif protein 25 (TRIM25) is an E3 ligase that ubiquitinates multiple substrates within the RLR signalling cascade and plays both RING (really interesting new gene)-dependent and RING-independent roles in RIG-I-mediated IFN induction. We report that the PRY-SPRY domain of TRIM25 interacts with the N-terminal extension (NTE) of DEAD-box helicase 3X (DDX3X), a host protein with multiple roles in RLR signalling. Gene reporter assays and knockdown studies reveal DDX3X and TRIM25 cooperate to activate the IFN- promoter following RIG-I activation independent of DDX3X’s catalytic activity. We also show that TRIM25 ubiquitinates DDX3X at several lysine residues in vitro and in cells.