A novel series of nickelates described by the formula Tl(Ln2Sr2)Ni2O9, Ln=La, Pr, Nd, Sm, Eu, and Gd, have been synthesized and their structures determined using both powder X-ray diffraction and powder neutron diffraction. The materials are isostructural with the superconducting cuprate, Tl(La2.4Ba1.6)Cu2O9, crystallizing in a tetragonal unit cell, space group I4/mmm, lattice parameters a=3.8069–3.7667(1) Å and c=30.055–29.390(1) Å. The compounds adopt the 1201–0201 intergrowth structure, consisting of layers of apex-sharing stoichiometric NiO6 octahedra connected by a TlO layer. A decrease in the cell dimensions is observed across the series, consistent with the lanthanide contraction and partial ordering of the strontium and lanthanide ions on the A cationic sites is observed. Magnetic susceptibility data reveal no evidence of long-range magnetic ordering for two of the compounds, Tl(La2Sr2)Ni2O9 and Tl(Gd2Sr2)Ni2O9.