Arsenic (As) polluted groundwater in Northern China was used as drinking water source and caused severe health problems. Characterization of As speciation is crucial to understand the health risk of As and its biogeochemical behaviors in groundwater. In this study, groundwater samples were collected from 26 wells in Northern China. Arsenic species in the groundwater were measured by high-performance liquid chromatography (HPLC) and inductively coupled plasma mass spectrometry (ICP-MS). Thioarsenate, one of the As species that is seldom reported in drinking groundwater, was detected in two-third of the sampling wells, even in the samples with total As concentration lower than 10 μg·L−1. Furthermore, the occurrence of thioarsenate in groundwater samples was dependent on the pH of groundwater, and thioarsenate was transformed to arsenite below a pH value of 8.2. The study demonstrated thioarsenate was prevalent in the alkaline drinking groundwater and would be a new As exposure pathway for the people and livestock living in the As-rich area.