Agenda 21 is a non-binding action plan of the United Nations with regard to sustainable development. It is a comprehensive plan of action to be taken globally, nationally, and locally by organizations of the United Nations System, governments, and major groups in every area in which humans impact the environment. The plan was a product of the Earth Summit held in Rio de Janeiro, Brazil, in 1992, and one of its major objectives was to achieve global sustainable development by 2000, with the "21" in Agenda 21 referring to the original target of the 21st century.

Agenda 21 is grouped into four sections, which are social and economic dimensions, conservation and management of resources for development, strengthening the role of major groups, and means of implementation. The implementation of Agenda 21 was intended to involve action at international, national, regional, and local levels. Some national and state governments have legislated or advised that local authorities take steps to implement the plan locally, as recommended in Chapter 28 of the document. These programs are often known as "Local Agenda 21" or "LA21".

Agenda 21 is a dynamic program that will be carried out by various actors according to the different situations, capacities, and priorities of countries and regions. The full implementation of Agenda 21, the Programme for Further Implementation of Agenda 21, and the Commitments to the Rio principles were strongly reaffirmed at the World Summit on Sustainable Development held in Johannesburg, South Africa, from 26 August to 4 September 2002.

Agenda 21 has been a subject of controversy and conspiracy theories, with some people claiming that it calls for governments to take control of all land use and not leave any of the decision-making in the hands of private property owners. However, these claims have been debunked, and Agenda 21 is a non-binding statement of intent and not a treaty.