Apoptosis signal‐regulating kinase 1 (ASK1) is a MAP kinase kinase kinase (MAPKKK) that activates the JNK and p38 MAP kinase cascades and is activated in response to oxidative stress such as hydrogen peroxide (H 2 O 2). A yeast two‐hybrid screening identified a serine/threonine protein phosphatase 5 (PP5) as a binding partner of ASK1. PP5 directly dephosphorylated an essential phospho‐threonine residue within the kinase domain of ASK1 and thereby inactivated ASK1 activity in vitro and in vivo. The interaction between PP5 and ASK1 was induced by H 2 O 2 treatment and was followed by the decrease in ASK1 activity. PP5 inhibited not only H 2 O 2‐induced sustained activation of ASK1 but also ASK1‐dependent apoptosis. Thus, PP5 appears to act as a physiological inhibitor of ASK1–JNK/p38 pathways by negative feedback.