"This post is too important to leave vacant any longer, especially during a war and a vital debate about U.N. reform," Mr. Bush said.
He called Bolton "one of America's most talented and valuable diplomats."
The move ends a five-month impasse with Democrats who accused Bolton of abusing subordinates and twisting intelligence to fit his conservative ideology.
Bolton went directly from the White House to the State Department where he was sworn in. Within five hours of his appointment, he arrived at the U.S. Mission in New York to begin work.
Bolton joined the president and Secretary of State Condoleezza Rice at the White House announcement ceremony. "It will be a distinct privilege to be an advocate for America's values and interests at the U.N. and, in the words of the U.N. charter, to help maintain international peace and security," he said.
Under the Constitution, the president may issue an appointment and bypass Senate confirmation when it is in recess. Such an appointment ends when the next session of Congress begins – January 2007, in this case.
Mr. Bush blames Democrats in the Senate for keeping the Bolton nomination from getting an up or down vote, which the White House is certain would have confirmed Bolton, reports CBS News Correspondent Mark Knoller.
Bush had refused to give up on Bolton even though the Senate had voted twice to sustain a filibuster against his nominee. Democrats and some Republicans had raised questions about Bolton's fitness for the job, particularly in view of his harsh criticism of the United Nations.
U.N. Secretary-General Kofi Annan welcomed Bolton's appointment and steered clear of the controversy over whether Bolton would be weakened by the recess appointment. "We look forward to working with him as I do with the other 190 ambassadors, and we will welcome him at a time when we are in the midst of major reform," Annan said.
Annan also pointedly noted that Bolton was one of many U.N. ambassadors. "I think it is all right for one ambassador to come and push," he said, "but an ambassador always has to remember that there are 190 others who will have to be convinced, or a vast majority of them, for action to take place."
Senate Democratic Leader Harry Reid of Nevada said Bolton was a "seriously flawed and weakened candidate." He said Bush "chose to stonewall the Senate" by using a recess appointment.
Senate Majority Leader Bill Frist, R-Tenn., said, "The president did the right thing by sending Mr. Bolton to the U.N. He is a smart, principled and straightforward candidate, and will represent the president and America well on the world stage."
Sen. Edward M. Kennedy, D-Mass., sharply criticized the move.
"It's a devious maneuver that evades the constitutional requirement of Senate consent and only further darkens the cloud over Mr. Bolton's credibility at the U.N," Kennedy said.
Republican Sen. George Voinovich of Ohio also said he was disappointed.
"I am truly concerned that a recess appointment will only add to John Bolton's baggage and his lack of credibility with the United Nations," Voinovich said.
A senior Republican said Bolton's appointment had been delayed long enough.
"If the president recess-appoints John Bolton, I can understand why because he's been waiting a long time to get the person that he believes is the best to represent his administration at the U.N," said Sen. Mitch McConnell, R-Ky.