Updated project metadata. Hydrogen peroxide (H2O2) signals regulate plant growth and defense by orchestrating genome-wide transcriptional re-programming, however, the specific mechanisms that regulate H2O2-dependent gene expression are poorly understood. Here we report the identification of the Mediator complex subunit MED8 as regulator of H2O2 responses, through a genetic screen of EAL4 promoter. Interestingly, the introduction of med8 mutation in the catalase-deficient background was associated with enhanced activation of salicylic acid biosynthesis and signaling pathways. Surprisingly, however, med8 seedlings were more tolerant to oxidative stress generated by methyl viologen. This tolerance phenotype is associated with the hyperactivation of defense and hormone signaling pathways in particular salicylic acid and jasmonic acid-related pathways. In addition, analysis of MED8 interactomes revealed interaction with novel pathways expanding Mediator complex function to processes beyond transcription, including miRNA biogenesis and and mRNA processing. We establish MED8 as a new component within the regulation of oxidative stress responses and demonstrate that MED8 act as negative regulator of H2O2 -driven activation of defense gene expression.