The structure of a synthetic analogue of Na-birnessite (NaBi) was studied by powder X-ray diffraction (XRD). It is shown that NaBi has a one-layer triclinic structure with sub-cell parameters: aP = 2.9513(4) Å, bP = 2.9547(4) Å, cP = 7.334(1) Å, αP = 78.72(2) Å, βP = 101.79(1) Å, γP = 122.33(1) Å, and space group P1̅ . This sub-cell is equivalent to the base-centered sub-cell with parameters: a = 5.174 Å, b = 2.848 Å, c = 7.334 Å, α = 90.53 Å, β = 103.20 Å, and γ = 90.07 Å. A structure model has been refined using the Rietveld technique. NaBi consists of vacancy-free Mn-bearing octahedral layers whose negative charge arises mostly from the substitution of Mn3+ for Mn4+. The departure from the hexagonal symmetry of layers results from Jahn-Teller distortion of Mn3+ octahedra, which are elongated along the a axis, segregated in Mn3+-rich rows parallel to the b axis, and separated from each other along the a axis by two Mn4+-rows. Structural sites of interlayer Na cations and H2O have been determined as well as their occupancies. The sub-cells of the two NaBi modifications described by Drits et al. (1997) as types I and II likely contain four sites for interlayer species, two of which are occupied by Na and the other two by H2O molecules. In the two NaBi varieties, these pairs of sites are split along the c axis and related by a center of symmetry. This splitting is consistent with the modulated structure of both NaBi types, which arises from the periodic displacement of interlayer species along the b axis with a periodicity λ = 6b (Drits et al. 1997).