The first examples of metal phosphite incorporating a squarate anion as a ligand have been synthesized by a hydrothermal method. Their structures contain zigzag chains of edge-sharing LnO(8) (Ln = Sm, Eu, and Gd) square antiprisms connected by phosphite groups to form 2D layers, which are pillared by squarate ligands into a 3D framework. The photoluminescence properties of the Eu compound have also been studied,