is a destructive ascomycete plant pathogen with worldwide distribution. Extensive research on different aspects of this pathogen's capability to cause disease will help to uncover clues about new ways to safely control Sclerotinia diseases. The thioredoxin (Trx) system consists of Trx and thioredoxin reductase (TrxR), which play critical roles in maintenance of cellular redox homeostasis. In this study, we functionally characterized a gene encoding a TrxR ( ) in . The amino acids of SsTrr1 exhibited high similarity with reported TrxRs in plant pathogens and targeted silencing of lead to a decrease in TrxR activities of mycelium. showed high expression levels during hyphae growth, and the levels decreased at the different stages of sclerotial development. gene-silenced strains produced a smaller number of larger sclerotia on potato dextrose agar medium. The observations were consistent with the inhibitory effects on sclerotial development by the TrxR inhibitor, anrunofin. The expression of showed a dramatic increase under the oxidative stress and the hyphal growth of gene-silenced strains showed more sensitivity to H O . gene-silenced strains also showed impaired virulence in different hosts. Taken together, our results suggest that encodes a TrxR that is of great important for oxidative stress tolerance, virulence, and sclerotial development of .