While local protectionism and market segmentation owing to fiscal decentralization are not conducive to broad economic development, they may be rational choices on a local scale. Based on a spatial Durbin model, we analyzed the relationship between environmental regulations and market segmentation in China using interprovincial panel data for 2004–2018. The results indicated that the “beggar-thy-neighbor” phenomenon persists in China; environmental regulations have a U-shaped impact on market segmentation, i.e., in most regions, environmental regulations strongly promote local market integration. Regions with greater decentralization are better able to promote local market integration through environmental regulation, suggesting that local governments are better able to compensate for market failures when vested with greater power. Hence, we propose that the central government should improve performance evaluation indicators for local governments and grant them greater autonomy; additionally, local governments should increase the intensity of environmental regulations as appropriate, thereby promoting both environmental protection and the unification of domestic markets.