Promoting regional coordination and strengthening inter-regional linkages are key tasks for China’s regional development during the 15th Five-Year Plan period. As a major regional strategy, the coordinated development of the Beijing-Tianjin-Hebei region aims to enhance the radiating capacity of the urban agglomeration and promote the development of the Bohai Rim and the northern hinterland. Based on prefecture-level panel data from the Bohai Rim during 2009—2023, this paper employs a spatial difference-in-differences model to evaluate the spillover effects of the Beijing-Tianjin-Hebei coordinated development strategy on peripheral areas. The results show that the strategy significantly promotes economic growth in peripheral areas and generates spatial spillovers that decline with increasing geographical distance. The effective driving range is mainly concentrated within 250 km of the Beijing-Tianjin-Hebei urban agglomeration, approximately forming a pentagonal area with Ulanqab, Chifeng, Dalian, Heze, and Jincheng as its vertices. Compared with Beijing or Tianjin as individual cores, the Beijing-Tianjin-Hebei urban agglomeration as a whole exhibits a broader driving effect. Mechanism analysis indicates that the strategy promotes peripheral economic growth through strengthened economic linkages, technology transfer, and industrial relocation. A functional polycentric urban structure further enhances these diffusion effects. Based on these findings, this paper suggests optimizing the spatial scope of policy implementation, improving the polycentric spatial network, and strengthening channels for factor mobility.