Despite extensive investigations, altering the regioselectivity of atom transfer radical addition (ATRA) to alkynes remains a highly desirable yet unachieved challenge. Guided by computational predictions, thiosulfonates were found herein as a tunable radical precursor for thiyl radicals instead of well-recognized sulfonyl radicals. Merging such a finding with ATRA to phenylacetylenes leads to a highly regio- and stereoselective approach to (E)-β-arylsulfonylvinyl sulfides. This protocol is feathered by mild conditions, low photocatalyst loading, no transition-metal catalyst required, and broad functional group compatibility. The successful application of our protocol in the late-stage functionalization of bioactive natural product derivatives demonstrates its synthetic utility. Mechanistic studies corroborate the photoredox-catalyzed ATRA pathway and reveal the pivotal role of thiyl radical, to which unprecedented regioselectivity was attributed.