ALG-2 (apoptosis linked gene-2) is an essential protein for the execution of apoptosis whose function is largely unknown. Here, we demonstrate that ALG-2 could interact with the C-terminus (amino acids 941-1375) of the apoptosis signal-regulating kinase 1 (ASK1) in BOSC23 cells as well as in vitro. ASK1 failed to bind to an isotype of ALG-2 found in the liver, ALG-2,1, in which two amino acids (Gly-121 and Phe-122) are deleted. This implies that the interaction is very specific. Cotransfection with ALG-2 resulted in the nuclear presence of ASK1 and inhibited the activation of c-Jun N-terminal kinase (JNK) by ASK1 in BOSC23 cells. This study reports that ALG-2 could regulate the subcellular localization and the JNK activity modulation of ASK1 by direct interaction.