The novel polymeric formates of general formula [(Fmd)-Ln(III)(HCOO4)(infinity), (Fmd(+) = NH2-CH+-NH2; Ln = Eu (1), Gd (2), Tb (3), Dy (4)] were synthesized through solvothermal methods in formamide solutions. The compounds are isotructural; they crystallize in the orthorhombic C222(1) chiral space group. The coordination geometry at the metal centers is square antiprismatic (coordination number eight), with each formate ligand bridging adjacent lanthanide ions. The overall negative three-dimensional (3D) framework charge is balanced by the formamidinium cations sitting inside the channels along the a axis, forming extensive N H center dot center dot center dot O hydrogen bonding with the surrounding cage. All the compounds have been characterized through single-crystal/powder X-ray diffraction, IR spectroscopy, and TG-MS analysis. Finally, their luminescence and magnetic properties have been assessed, leading remarkable emission intensities, especially for the Tb(III) compound (Phi = 0.83), with corresponding lifetime decays in the micro (Dy) and millisecond (Tb, Eu) time scale. A weak but sizable antiferromagnetic interaction has been observed for the Gd(III) derivative.