||Maintaining a single active X chromosome is essential for the development of female mammals. In pre-implantation embryos of mice, the expression of Xist triggers X chromosome inactivation (XCI) in cis, is limited to the paternal allele and maternal Xist (Xm-Xist) is repressed. Here, we demonstrate for the first time that trimethylation of histone H3 lysine residue 9 (H3K9me3) prevents induction of maternal Xist activation by RNF12 to establish imprinted XCI during embryo pre-implantation. Ectopic expression of H3K9me3 demethylase encoded by Kdm4b derepresses Xist expression from Xm in the same developmental window as Xp-Xist activation. Xm-Xist expression establishes global XCI at the blastocyst stage and causes marked suppression of X-linked genes. Thus, we conclude that imprinting by H3K9me3 silences Xm-Xist in early pre-implantation to establish imprinted XCI.