XIAP is known as a potent inhibitor of apoptosis, but in addition is involved in cellular signalling, including the NFκB, JNK and TGFβ pathways. Our search for XIAP-interacting partners led us to Siva1, a proapoptotic protein that is known to play a role in T-cell apoptosis through a caspase-dependent mitochondrial pathway. The interaction sites between XIAP and Siva1 were mapped to the RING domain of XIAP and the N-terminal, SAH-containing and death-homology-region-containing domains of Siva1. Co-immunoprecipitation experiments showed that XIAP, Siva1 and TAK1 form a ternary complex in Jurkat T cells. Reporter-gene analysis revealed that Siva1 inhibits XIAP- and TAK1-TAB1-mediated NFκB activation. By contrast, Siva1 increased XIAP- and TNFα-mediated AP1 activity and prolonged TNFα-induced JNK activation, whereas knock down of Siva1 resulted in reduced JNK activation. This suggests that Siva1 differentially modulates signalling by JNK and NFκB and shifts the balance between these pathways towards enhanced JNK activation, a situation that promotes apoptosis. Ectopically expressed Siva1 increased caspase-3 activity, which was inhibited by XIAP in a ubiquitin-ligase-dependent manner. In line with this, Siva1 was lysine-48-linked polyubiquitylated by XIAP. Our findings suggest that, via physical interaction with XIAP and TAK1, Siva1 diminishes NFκB and enhances JNK activity to favour apoptosis.