S-adenosylhomocysteine (SAH), formed after donation of the methyl group of S-adenosylmethionine (SAM) to a methyl acceptor, is reversibly hydrolyzed to adenosine (ADO) and homocysteine (HCY) by S-adenosylhomocysteine hydrolase (SAHH). In chestnut blight fungus (Cryphonectria parasitica), sahh, a hypovirus-regulated gene that encodes a deduced SAHH protein was shown to have an SAHH enzymatic activity in vitro. Deletion of sahh resulted in the increased accumulation of intracellular SAH and SAM but decreased ADO, and a remarkably increased accumulation of transcripts that encode adenosine kinase, methionine adenosyltransferase, and an O-methyltransferase, key components of the methylation pathway. The Δsahh knockout mutants showed a phenotype of slower growth rate, fewer aerial hyphae, loss of orange pigment, absence of asexual fruiting bodies and conidia, and a significant reduction in virulence. Deletion of sahh significantly reduced the accumulation level of transcripts of the cyp1 that encodes cyclophilin A as well as genes of the heterotrimeric G-protein signaling pathways including cpga1, cpgb1, and cpgc1 and ste12, a target activated by the MAP kinase cascade. Taken together, we demonstrated that SAHH is required for virulence and multiple traits of phenotype in C. parasitica, by regulation of the expression of genes involved in key process of the cell.