Copper(I) iodide reacts with 1,2-bis(diphenylselenophosphinyl)ethane, [Ph₂P(Se)(CH₂)₂P(Se)Ph₂, dppeSe₂] in a 2 : 1 molar ratio in acetonitrile–chloroform to form an infinite polymer, [Cu₄I₄{Ph₂P(Se)(CH₂)₂P(Se)Ph₂}₂]_n (1), characterised using analytical data, infrared spectroscopy and X-ray crystallography. The polymer contains the centrosymmetric repeat unit Cu₄I₄{Ph₂P(Se)(CH₂)–}₄ (A), with four copper atoms forming a step-like (zig-zag) chain, Cu(2)⋯Cu(1)⋯Cu(1)*⋯Cu(2)*. While the Cu(1)⋯Cu(2) distance of 2.495(2) Å is shorter than the corresponding van der Waals separation (2.80 Å), the Cu(1)⋯Cu(1)* distance is longer at 2.820(4) Å. The Cu₄I₄ core has two doubly bridging and two triply bridging iodine atoms, with Cu–I bond lengths ranging from 2.514(2) to 2.910(2) Å. The terminal copper atoms, Cu(2), are bonded to two Se atoms from two Ph₂P(Se)CH₂– moieties [Cu(2)–Se(2) 2.359(2), Cu(2)–Se(1) 2.550(2) Å]. One of these Se atoms, Se(1), bridges Cu(1) and Cu(2) [Cu(1)–Se(1) 2.706(2) Å]. The four pendant Ph₂P(Se)CH₂– moieties of the repeat unit A link to four other Cu₄I₄ cores, leading to the formation of the polymer 1, which is the first example in tertiary phosphine chalcogenide chemistry with a “tetradentate” mode of Ph₂P(Se)(CH₂)₂P(Se)Ph₂ coordination.