Thioredoxin 1 (Trx1) facilitates the reduction of signaling molecules and transcription factors by cysteine thiol-disulfide exchange, thereby regulating cell growth and death. Here we studied the molecular mechanism by which Trx1 attenuates cardiac hypertrophy. Trx1 upregulates DnaJb5, a heat shock protein 40, and forms a multiple-protein complex with DnaJb5 and class II histone deacetylases (HDACs), master negative regulators of cardiac hypertrophy. Both Cys-274/Cys-276 in DnaJb5 and Cys-667/Cys-669 in HDAC4 are oxidized and form intramolecular disulfide bonds in response to reactive oxygen species (ROS)-generating hypertrophic stimuli, such as phenylephrine, whereas they are reduced by Trx1. Whereas reduction of Cys-274/Cys-276 in DnaJb5 is essential for interaction between DnaJb5 and HDAC4, reduction of Cys-667/Cys-669 in HDAC4 inhibits its nuclear export, independently of its phosphorylation status. Our study reveals a novel regulatory mechanism of cardiac hypertrophy through which the nucleocytoplasmic shuttling of class II HDACs is modulated by their redox modification in a Trx1-sensitive manner.