The sex chromosomes of birds are designated Z and W. The male is homogamous (ZZ), and the female is heterogamous (ZW). The chicken W chromosome is a degenerate version of the Z chromosome and harbors only 28 protein-coding genes. We studied the expression pattern of the W chromosome geneMIER3(showing differential expression during gonadogenesis) in chicken embryonic gonads and its potential role in gonadal development. The W copy ofMIER3(MIER3–W) shows a gonad-biased expression in chicken embryonic tissues which was different from its Z copy. The overall expression ofMIER3–WandMIER3–ZmRNA and protein is correlated with the gonadal phenotype being higher in female gonads than in male gonads or female-to-male sex-reversed gonads. Chicken MIER3 protein is highly expressed in the nucleus, with relatively lower expression in the cytoplasm. Overexpression ofMIER3–Win male gonad cells suggested its effect on the GnRH signaling pathway, cell proliferation, and cell apoptosis.MIER3expression is associated with the gonadal phenotype.MIER3may promote female gonadal development by regulatingEGR1andαGSUgenes. These findings enrich our knowledge of chicken W chromosome genes and support a more systematic and in-depth understanding of gonadal development in chickens.