A lamellar neptunium(IV) methylphosphonate, Np(CH3PO3)(CH3PO3H)(NO3)(H2O)center dot H2O, has been prepared under hydrothermal conditions via the in situ reduction of Np-VI to Np-IV. The single crystal structure of this compound shows polar layers that are joined to one another via a hydrogen-bonding network involving interlayer water molecules. Magnetic susceptibility measurements demonstrate that the Np-IV ions are magnetically isolated from one another.