Topochemical modification methods for solids have shown great potential in generating metastable structures inaccessible through classical synthetic routes. Here, we present the enhanced topotactic reduction of the multiferroic compound YMnO3. At moderate temperature in ammonia flow, the most reduced YMnO3-δ (δ = 0.5) phase could be stabilized. XRD, PND, and HREM results show that phase separation occurs into two intimately intergrown layered sublattices with nominal compositions ∞[YMn2+O2+x](1-2x)+ and ∞[YMn2+O3-x](1-2x)- containing versatile Mn2+ coordinations. The former sublattice shows original AA stacking between Mn layers, while AB stacking in the latter results from oxygen removal from the parent YMnO3 crystal structure.