The blue α-polymorph of the orthoperiodato-cuprate(II) K₃[CuIO₆]·4H₂O is synthesized in a polytetrafluoroethylene-lined autoclave at 150 °C by reacting copper(II) nitrate and potassium periodate in aqueous 30 m potassium hydroxide solution. The monoclinic crystal structure (space group P2₁/c) consists of (Formula presented.) CuIO₆]^3– chains, which are separated by K⁺ cations and water molecules. The copper(II) cation is in square planer coordination by oxygen atoms of two fully deprotonated periodate octahedra. The (Formula presented.) (CuO_4/2)(IO_4/2O_2/1)]^3– chain contains both the Δ and the Λ enantiomers. At the higher temperature of 200 °C and with an excess of potassium periodate, violet to purple crystals of β-K₃[CuIO₆]·4H₂O precipitate. The monoclinic structure (space group P2₁/m) comprises chains (Formula presented.) (CuO_2/3O_2/2)(IO_1/3O_2/2O_3/1)]^3–, which have the same composition as those of the α-form but a different internal connectivity. K₃[CuIO₆]·4H₂O is dehydrated above 100 °C. The α-polymorph is paramagnetic down to 1.8 K, and the magnetic moment aligns with the d⁹ configuration of copper(II). The β-form, which allows for direct as well as super-exchange between the moments in the linear spin-½ chain, shows a highly complex magnetism that indicates antiferromagnetic coupling and magnetic frustration.