The UN Transitional Administration in East Timor provided an interim civil administration and a peacekeeping mission in the territory of East Timor. Its responsibilities included providing a peacekeeping force to maintain security and order; facilitating and coordinating relief assistance to East Timorese; facilitating emergency rehabilitation of physical infrastructure; administering East Timor and creating structures for sustainable governance and the rule of law; and assisting in the drafting of a new constitution and conducting elections. It was led by Sergio Vieira de Mello.

A coalition of nations, led by Australia, sent troops to support the peace keeping mission. Countries supporting this mission included Argentina, Brazil, Malaysia, Singapore, Thailand, Portugal, United Kingdom, Sweden, Ireland, Denmark, and New Zealand. While the United States supported the transition authority, it did so mainly by underwriting contracts to replace destroyed infrastructure and thus avoided a military involvement.

UNTAET was established on October 25, 1999 and was abolished on May 20, 2002 with most functions passed to the East Timor government. The military and police forces were transferred to the newly created United Nations Mission of Support to East Timor .

See Timeline of UN peacekeeping missions.