Regional transport is one of the major causes of severe haze pollution in the Beijing–Tianjin–Hebei (BTH) region. However, the process of pollutant transport and the specific mechanisms that affect haze pollution in the BTH region remain unclear. In this study, the WRF-Chem model was used to quantify the contribution of regional transport to a severe pollution event over the BTH region that occurred between October 29 and November 6, 2015. The results show that the regional transport of pollutants from the area south of the BTH region, which was induced by synoptic forcing and topographic effects, contributed up to 70% of the PM_2.5 concentration in the BTH region. The peak PM_2.5 concentration closely corresponded to a light southerly wind, high relative humidity, low planetary boundary layer height, and weak turbulent kinetic energy. The pollutants derived from regional transport were located mainly at an altitude of 0.5–1.5 km during the accumulating stage and at 0–3 km during the severely polluted stage, which reflects mainly the height of the inversion layer and the strength of the wind, whereas pollution derived from local emissions was restricted mainly to the near-surface layer (<500 m). This study highlights that, in addition to local emission-reduction strategies, multi-provincial cooperative efforts will be necessary to improve air quality in the BTH region.