Atom-efficient organoactinide-catalyzed intermolecular hydrothiolation of terminal alkynes is achieved by Th(IV) and U(IV) complexes to yield vinyl sulfides. The conversion is highly Markovnikov selective and is capable of utilizing aromatic, benzylic, and aliphatic thiols. Kinetic measurements of the transformation mediated by Me2SiCp2 '' Th[CH2(TMS)](2)(1a; Cp '' = eta(5)-Me4C5) shows the reaction is zero-order in [thiol], first-order in [1a], first-order in [alkyne] at lower alkyne concentrations, and zero-order at higher [alkyne]. The proposed catalytic cycle invokes turnover-limiting insertion of the alkyne into the Th-S bond followed by a rapid Th-C bond protonolysis. All reactions except entries 7 and 8 are quantitative by H-1 NMR.