The　development　of　visible　light-regulated　polymerizations　for　precision　synthesis　of　polymers　has　drawn　considerable　attention　in　the　past　years.　In　this　study,　an　ancient　dye,　indigo,　is　successfully　identified　as　a　new　and　efficient　photoacid　catalyst,　which　can　readily　promote　the　ring-opening　polymerization　of　lactones　under　visible　light　irradiation　in　a　well-controlled　manner,　affording　the　desired　polyester　products　with　predictable　molecular　weights　and　narrow　dispersity.　The　enhanced　acidity　of　indigos　by　excitation　is　crucial　to　the　H-bonding　activation　of　the　lactone　monomers.　Chain　extension　and　block　copolymer　synthesis　are　also　demonstrated　with　this　method.　©　2024　Wiley-VCH　GmbH.