Skeletal editing of aromatic heterocycles represents a straightforward approach to rapidly expand the accessible chemical space. While notable progress has been made on the direct modification of various nitrogen- and oxygen-heterocycles, editing of prevalent sulfur-containing heteroarenes, especially for single-atom insertion, remains exceedingly rare. This disparity is primarily attributed to the sulfur atom's inherent nucleophilicity and high susceptibility to oxidation. Here we present a conceptually distinct photocatalytic strategy that enables the insertion of a boron atom into a diverse range of thiaarenes, furnishing previously inaccessible cyclic thioborane scaffolds and synthetically valuable alkyl boronates in an efficient manner. Furthermore, mechanistic studies have revealed that the boron insertion step proceeds through an unprecedented mechanism.