The most widely used accelerators for microvia filling by copper electroplating are 3-mercapto-1-propanesulfonate (MPS) and bis(3-sulfopropyl) disulfide (SPS). In this study, a new accelerator, 3,3-thiobis(1- propanesulfonate) (TBPS), which has neither a thiol group (e.g., MPS) nor a disulfide group (e.g., SPS) but rather contains a thioether group and two sulfonic acid groups, was investigated and formulated to perform microvia filling by copper electroplating. The accelerating effect of TBPS on copper electrodeposition and its chemical interaction with chloride ions and H2SO4 were characterized by galvanostatic measurements. Electrochemical analyzes showed that the acceleration of TBPS on copper electrodeposition strongly depended on the chloride ion and H2SO4 concentrations. An optimal copper plating solution composed of CuSO4, polyethylene glycol (PEG), TBPS, chloride ions, and H2SO4 was developed and used to perform bottom-up copper filling of microvias. Although TBPS contains a thioether group inste of a thiol group, a trace amount of TBPS can strongly accelerate copper electrodeposition in the presence of proper chloride ion and H2SO4 concentrations.