Three new complexes of 1,3-diethyl-thiobarbituric acid (HDetba), barbiturate-bridged 2D Mn(II) polymer [Mn(H2O)2(Detba)2] (1), trinuclear [Mn3(H2O)10(Detba)6] (2) and discrete molecular [Mn(H2O)4(Detba)2]·H2O (3) are synthesized and structurally characterized by the X-ray single crystal technique. In 1–3, the Detba− ions are coordinated to Mn(II) only through O atoms with manganese ions in the octahedral environment. In 1, the Mn(II) ion is coordinated by four μ2 bridge Detba− ions and two terminal H2O molecules. In 2, the Mn1 ion is coordinated by three terminal H2O molecules, two terminal and one μ2 bridge Detba− ion, and Mn2 is connected with two μ2-Detba− ions and four H2O molecules. In 3, the Mn(II) ion is coordinated by two terminal Detba− ions and four terminal H2O molecules. There are intermolecular hydrogen bonds O–H⋯O, O–H⋯S in the structures of 1–3 which form the 3D networks. Structure 2 is stabilized by the π–π interaction. The compounds thermal decomposition comprises dehydration steps and the organic ligand oxidation.