Master sex determining genes governing genetic sex determination have only been characterized in a very few non-mammalian vertebrates species. Teleost fishes represent the largest group of vertebrates on earth and are known for their extreme diversity with regards to sex determination. Among teleosts, all salmonid species investigated so far have a genetically determined sex with a male heterogamety. However, the master sex determining gene governing this strict genetic control in salmonid is still unknown. Here we found a previously unknown Y-chromosome gene specifically expressed during testicular differentiation in rainbow trout. This gene (named sdY for Sexually Dimorphic on the Y) was characterized by next-generation sequencing of male and female embryonic gonads. Its encodes a protein that is only found in salmonids and displays similarities with the C terminal domain of IRF9, a member of the Interferon Regulatory Factor family. We also demonstrated that sdY is conserved as a male-specific genomic sequence in all salmonids. This wide conservation of sdY -linkage with the heterogametic sex of salmonids was rather unexpected as these species do not share any Y chromosome conserved synteny. Because of its wide Y-linked conservation in salmonids and its pattern of expression restricted to the differentiating testis, our results provide strong evidence that sdY is a conserved salmonid master sex determining gene.