Dioxins elicit a wide range of toxic responses in a species- and tissue-specific manner. Epigenetic dysregulation may underlie some of the toxicological outcomes of exposure to dioxins. In this regard, we show for the first time that the most toxic dioxin congener, 2,3,7,8-tetrachlorodibenzo-ρ-dioxin (TCDD), administered at 3 µg/kg bodyweight induces Cyp1a1 promoter demethylation in the adult mouse liver within 24 h post-exposure. In line with an active demethylation mechanism, the demethylation was coincident with changes in 5-hydroxymethylcytosine and recruitment of the active demethylation mediator proteins Tet3, Tdg and Apex1. As anticipated, there were increases in the transcriptionally competent trimethylation of H3 lysine 4 and H4 pan-acetylation. These epigenetic modifications persisted up to 40 days post exposure, and repeat administration of TCDD (100 ng/kg) to the pre-treated animals resulted in a three-fold superinduction of Cyp1a1 mRNA, showing that there was epigenetic memorization of the initial exposure event. The epigenetic outcomes of TCDD administration were Ahr-dependent, as Ahr deficient mice and isolated hepatocytes had intact Cyp1a1 promoter epigenetic configuration characterized by absence of the DNA demethylation factors. Also, TCDD did not alter the mRNA expression levels of the active DNA demethylation mediator genes including Tet1, Tet2, Tet3, Tdg, Apex1, Gadd45a and Apobec1. Our results indicate that dioxin induces a rapid onset of durable epigenetic alterations that influence response to subsequent exposures, and are thus toxicologically significant.