The South Asian countries share many commonalities in terms of environmental problems and opportunities. Many of them share natural boundaries, biodiversity hotspots, inter-state water resources. The Himalayas influence the confluence of cultural and religious practices of Himalayan states. They also share socio-political and economic interests in many ways. Again, most of these countries were affected by colonial exploitation of their natural capital and they strive to come out of the impact of colonialism after independence. In this context, the Constitutions of these countries play a decisive role in shaping and directing their developmental priorities and environmental mandate. Many a time, they use the Constitution as a tool to protect the environment. The environmental judicial activism through public interest litigation in India, Sri Lanka, and Pakistan is a case in point. The Constitutions of all the South Asian Countries contain environmental obligations that pave the way for good environmental governance. Environmental constitutionalism provides a constitutional mandate to the State to protect and improve environment besides rolling out a rights-based approach for the citizens to live in a healthy environment. Against this backdrop, this chapter analyses the strategy of environmental constitutionalism in South Asian countries and examines their effectiveness and reach in shaping the pace of environmentally sustainable development in the South Asian region. This chapter also argues the relevance of ‘thick’ environmental constitutionalism vis-a-vis ‘thin’ environmental constitutionalism and suggests a strong regional cooperation through South Asian Association for Regional Cooperation (SAARC) and other initiatives.