With their Janus-faced property of being at once dynamic and stable, epigenetic mechanisms have for long been proposed to act as potential molecular mnemonics, but a cell type-specific, locus-restricted and temporally controllable interrogation thereof has thus far been lacking. Over the past years, accumulating evidence has shown that memories are likely encoded in sparse populations of brain cells, so-called engrams, which have, with a few exceptions, received little molecular attention. Here, we combine c-Fos driven engram tagging technologies with CRISPR-based epigenetic editing in vivo to assess whether and the extent to which a locus-specific epigenetic regulation within engram cells can contribute to memory formation and storage. Focusing on the promoter region of Arc, a master regulator of learning and synaptic plasticity, we find that its epigenetic regulation in the mouse hippocampus is necessary and sufficient for both memory expression after learning and for memory maintenance after recall. Furthermore, such epigenetic editing and with it the behavioral consequences is reversible, and can bidirectionally alter memory capacities even outside the initially labile phase of memory consolidation. Together, these findings indicate that epigenetic mechanisms causally contribute to mnemonic processes.