The wetness index (W) was calculated using gridded monthly precipitation and potential evapotranspiration (PET) in the Beijing–Tianjin–Hebei region of China. Meteorological data from 1991 to 2020 were provided by the Climate Research Unit. The change in W and the factors influencing it were analyzed at regional and urban levels. The results show that (1) W of the Beijing–Tianjin–Hebei region fluctuated greatly year by year. W decreased (p < 0.05) during the years from 1991 to 2000, rose slightly (p < 0.05) from 2001 to 2010, and experienced almost no change in the last decade. (2) In the last two decades, W in some areas of the Beijing–Tianjin–Hebei region increased, probably owing to the increase in precipitation in this area. (3) In the context of climate warming, W values in the Beijing–Tianjin–Hebei region and major cities are affected by precipitation, while temperature has little impact on the values.