The first example of near-room-temperature alpha-arylation of benzo[b]thiophenes is reported. The discovery rests on the observation of a switch in alpha-/beta-regioselectivity at different loadings of Pcl(2)(dba)(3)center dot CHCl3 in the coupling between benzo[b]thiophene and 4-iodotoluene. We show that this unprecedented regioselectivity switch is driven by a Ag(I)-mediated C-H activation at the alpha-C-H position, which becomes the dominant mode of reactivity at low concentrations of Pd. Competition experiments, kinetic studies, KIE, and D/H scrambling experiments have been carried out supporting this mechanism.