The move came after a protracted delay in which senators said Bolton would hurt US credibility and had stalled his nomination.
"This post is too important to leave vacant any longer, especially during a war and a vital debate about UN reform," Bush said on Monday during a joint public appearance with Bolton, whom he said had "my complete confidence".
Opposition Democrats had used procedural delaying tactics to block a confirmation vote on Bolton, who can now serve at the world body until a new US Congress convenes in January 2007 after elections in November 2006.
The critics had pointed to Bolton's unabashed anti-UN statements in the past as well as his harsh management style as undersecretary for arms control and international security in Bush's first term.
Faced with procedural delaying tactics the administration's refusal to hand over documents linked to his State Department service, Bush's Republicans never mustered the 60 votes necessary for ending debate on the nomination.
Bush bypassed the Senate to
make the appointment
"America has now gone more than six months without a permanent ambassador to the United Nations," said Bush.
"So today I've used my constitutional authority to appoint John Bolton to serve as America's ambassador to the United Nations."
UN reform debate
Bolton's appointment comes at a time when the world body embarks on a debate about sweeping reforms, including adding permanent members to the UN Security Council.
"Ambassador Bolton will work to build on that progress by helping the UN continue to find effective new ways to match its good intentions with good results," said Bush.
"I am prepared to work tirelessly to carry out the agenda and initiatives that you and Secretary [of State Condoleezza] 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 Bolton.
Democratic Senator John Kerry, Bush's rival for the White House in 2004, acknowledged Bush's constitutional right to make such an appointment but condemned the move as "the wrong decision".
"It only diminishes John Bolton's validity and leverage to secure America's goals at the UN," Kerry said in a statement. "This is not the way to fill our most important diplomatic jobs."