A new variety of ammonium lanthanum sulfate, β-(NH4)La(SO4)2, was synthesized hydrothermally. The crystal structure was solved ab initio from powder diffraction data collected using a conventional X-ray source. The powder diffraction pattern was indexed by the successive dichotomy method: The symmetry is monoclinic, space group Pn, cell dimensions a=6.9365(4) Å, b=9.0055(5) Å, c=5.4541(4) Å, β=90.672(8)°, V=340.68(3) Å3 and Z=2 [M20=65, F30=124(0.0071,34)]. Direct methods were used for structure solution, and refinement of the atomic coordinates was carried out by the Rietveld method (RF=0.061, Rwp=0.099). The structure contains anionic layers built from lanthanum atoms ninefold coordinated to monodentate and bidentate sulfate oxygen atoms. Ammonium groups, which provide hydrogen bonds, are located between the layers. The crystal structure contrasts with that of the α phase, which is less dense by a factor of 4.4%.