Thiols　are　well-established　hydrogen　atom　transfer　catalysts　for　organic　reaction　development　and　agents　for　polymer　modification　and　cross-linking.　Herein,　we　demonstrate　a　different　scenery　that　aryl　thiols　like　1-naphthalenethiol　and　1-pyrenethiol　(PySH)　can　also　act　as　a　photocatalyst　for　polymerization,　which　can　be　photoactivated　under　visible　light　to　function　as　a　photoacid　catalyst　to　promote　the　ring-opening　polymerization　of　lactones,　affording　well-defined　polymers　with　controlled　molecular　weights　and　narrowed　dispersities　(&　Dstrok;　＜　1.10).　Remarkably,　this　photoacid　could　even　promote　the　cationic　polymerization　of　vinyl　ether,　which　usually　requires　an　acid　stronger　than　sulfonic　acids.　Moreover,　PySH　can　also　serve　as　a　photoredox　catalyst　for　the　atom　transfer　radical　polymerization　(ATRP)　of　methyl　methacrylate.　The　increased　acidity　is　supported　by　further　investigation　of　the　photophysical　and　photoacid　properties　of　PySH　through　combined　experimental　and　theoretical　studies.