“This post is too important to leave vacant any longer, especially during a war and a vital debate about UN reform,” Mr Bush said.
During a public appearance with Mr Bolton, Mr Bush said he had “my complete confidence.”
Mr Bolton, a staunch critic of the UN, has been appointed at a time when the world body is embarking on a debate about sweeping reforms, including new permanent UN Security Council members.
The UN also faces possible debates over Iran’s and North Korea’s nuclear programs.
Opposition Democrats had used procedural delaying tactics to block a confirmation vote on Mr Bolton.
He can now serve at the world body until a new US Congress convenes in January 2007 after elections in November 2006.
Critics had pointed to Mr Bolton’s unabashed anti-UN statements, his harsh management style as undersecretary for arms control and international security during Mr Bush’s first term, and charges that he pressured intelligence analysts to support his views.
Mr Bush’s Republicans in the Senate never mustered the 60 votes needed to break through procedural delaying tactics that indefinitely put off a final confirmation vote on the controversial nomination.
“America has now gone more than six months without a permanent ambassador to the United Nations,” said Mr Bush.
“So today I’ve used my constitutional authority to appoint John Bolton to serve as America’s ambassador to the United Nations,” he said
“I am prepared to work tirelessly to carry out the agenda and initiatives that you and (US) Secretary (of State Condoleezza Rice) Rice direct. We seek a stronger, more effective organisation, true to the ideals of its founders, and agile enough to act in the 21st century,” said Mr Bolton.
UN Secretary General Kofi Annan said he looked forward to working with Bolton.
“We will work with him as we worked with other US permanent representatives,” said Mr Annan.
But he warned that “an ambassador always has to remember that there are 190 others who will have to be convinced or the vast majority of them, for action to take place.”
Mr Bolton was sworn in shortly after Mr Bush’s announcement and travelled immediately to New York to take up his duties, officials said.