Achim Steiner (born 1961 in Brazil) is a German expert in environmental politics. From 2001 to 2006 he was Director General of the International Union for Conservation of Nature and Natural Resources (IUCN). Since June 2006 he has been Executive Director of the United Nations Environment Programme (UNEP).

Steiner grew up in Brazil and was educated in England at Dover College. He studied philosophy, political science and economics at the University of Oxford. He received a Master of Arts degree in Economics and regional planning from the University of London, specializing in development economics, regional planning, and international development and environment policy. He also was a visiting scientist at German Development Institute in Berlin and at Harvard Business School. After leaving university Steiner worked for several local environmental organisations before he worked for IUCN in Washington, D.C. and Asia. As a Senior Policy Adviser of the IUCN Global Policy Unit in Washington D.C., he spearheaded the development of new partnerships between the environmental community and the World Bank and United Nations system. In South-East Asia, he worked on a programme for sustainable management of Mekong River watersheds and community-based natural resources management. In 1998 he became Secretary-General of the World Commission on Dams (WCD) in Cape Town, South Africa. In that capacity, he managed a global programme of work to bring together the public sector, civil society, and the private sector in a global policy process on dams and development.

In 2001 he returned to IUCN as Director-General, where he was responsible for managing and overseeing 1,000 staff stationed in 42 countries.