The growth in vertebrates is controlled by growth hormone (GH) gene which is a member of the somatotropin/prolactin family. In salmonids, a series of gene duplications is thought to have occurred in GH gene. In this family, the diversity in intron and exon regions of the GH gene is assumed to be not only in charge of a different growth rate but also in sex maturation. Due to the sex determination importance in salmonids, especially during early stages of development, in present study, an easy and accurate molecular marker was investigated for sex determination of Oncorhynchus mykiss fish using the diversity of the exon and intron 5 of GH gene. A pseudogene (GH‐ψ) copy of growth hormone gene was found on the Y chromosome which is in homology with type‐2 GH genes that can be used for sexual dimorphism of this species. This pseudogene was detected using a specific primer pairs combination using polymerase chain reaction (PCR) to select monosex female population. As recent findings suggest new functions of pseudogenes in the regulation of gene expression, it is believed that the putative gene could have an impact on GH expression in this species.