This article explores resource-based downstream diversification, critical minerals and industrial policy in selected Southeast Asian countries. It provides an analysis of both the historical and contemporary experience with resource-based development and diversification, focusing primarily on the increasingly important critical minerals sector and related downstream industries. The article explores both successful and unsuccessful experiences across three distinct dimensions: (1) adding value to raw minerals, (2) the development of mineral-intensive manufacturing industries, and (3) the role of regional integration, and analyses the key policies pursued and lessons learned. Through several case studies spanning various countries, minerals and technologies, the article provides key insights and considerations for policymakers everywhere that are looking to add value to their natural resources, create mineral-intensive downstream manufacturing industries and diversify their economies.