Visible-light-induced direct C-H arylation of S,S-functionalized internal alkenes, that is, α-oxo ketene dithioacetals and analogues, has been efficiently realized with aryldiazonium salts (ArN2BF4) as coupling partners and Ru(bpy)3Cl2·6H2O as photosensitizer at ambient temperature. The strategy to activate the internal olefinic C-H bond by both the alkylthio and electron-withdrawing functional groups was investigated. The synthetic protocol was successfully applied to the synthesis of all-carbon tetrasubstituted alkenes including tamoxifen.