China's Belt and Road Initiative (BRI) is expected to be the largest infrastructure development scheme of the twenty-first century. There is escalating concern over BRI's potential environmental impacts in Southeast Asia, a global biodiversity hotspot and a focus area of BRI development. Case studies of Indonesia, Myanmar, Lao PDR and Malaysia show that the success of BRI in bringing about sustainable growth and opportunities depends on the Chinese government and financiers, as well as the agencies and governments involved when BRI investments take place. The adoption of best environmental practices is critical in ensuring that growth is sustainable and that bad environmental practices are not locked in for decades to come.