In a major victory for US conservatives, the deeply-divided chamber voted 52 to 48, largely along party lines in the Republican-controlled US senate.
Kamala Harris has condemned the confirmation of Amy Coney Barrett to the Supreme Court as "despicable".
President Donald Trump revelled in one of his signature achievements at a White House ceremony to celebrate Justice Barrett's nomination, eight days before the election.
The made-for-TV prime-time event on the White House lawn on Monday night mirrored one a month ago, when Justice Barrett’s nomination was announced, which preceded a coronavirus outbreak among top Republicans including Mr Trump himself.
It came little more than an hour after the Republican-controlled Senate confirmed Ms Barrett to the lifetime appointment on a 52-48 vote, with Democrats unified in opposition.
Taking to Twitter, Ms Harris said Republicans had "denied the will of the American people" by pushing the nomination through.
Justice Barrett's confirmation as successor to liberal Justice Ruth Bader Ginsburg, who died last month, creates a 6-3 conservative majority on the high court. One Republican, Susan Collins, voted against the confirmation.
Mr Trump, who has been touting the appointment at campaign rallies to the cheers of his supporters, had pressed the Senate to confirm Justice Barrett, 48, before the 3 November election in which he trails Democrat Joe Biden in national opinion polls. No Supreme Court justice had ever been confirmed so close to a presidential election.
“The Barrett family has captured America’s heart. It is highly fitting that Justice Barrett fills the sea of a true pioneer for women, Justice Ruth Bader Ginsburg,” Mr Trump said with a smiling Justice Barrett at his side.
In contrast to the White House event last month, more people wore masks and seats were spread out to ensure social distancing. Several of the Republican senators who voted to confirm Justice Barrett were in attendance, although not Majority Leader Mitch McConnell, who has not been to the White House since August due to COVID-19 concerns.
Conservative Supreme Court Justice Clarence Thomas, wearing his judicial black robes, administered one of the two oaths of office that justices have to take.
Judge Barrett vows independence
In brief remarks, Justice Barrett declared her independence from Mr Trump and the political process even as the president stood behind her.
“The oath that I have solemnly taken tonight means at its core I will do the job without fear or favor and do it independently of the political branches and of my own preferences,” she said.
After the ceremony, Mr Trump and Justice Barrett waved to applauding guests from the White House balcony.
Chief Justice John Roberts will administer the separate judicial oath at the court on Tuesday, the court said in a statement.
Justice Barrett is expected to participate in arguments on 10 November in a case in which Mr Trump and Republican-led states are seeking to invalidate the Affordable Care Act. The 2010 healthcare law, also known as Obamacare, has helped millions of Americans obtain medical insurance and barred private insurers from denying medical coverage to people with pre-existing conditions.
Justice Barrett has criticised previous rulings upholding Obamacare but said during her confirmation hearing she had no agenda to invalidate the measure.
During her confirmation hearing before the Senate Judiciary Committee two weeks ago, Justice Barrett, a favourite of Christian conservatives, irked Democrats by sidestepping questions on abortion, presidential powers, climate change, voting rights, Obamacare and other issues.
The 26 September Rose Garden ceremony at which Mr Trump named Justice Barrett as his nominee preceded a wave of COVID-19 cases among top Republicans including Mr Trump and first lady Melania Trump. The president spent three nights hospitalised receiving COVID-19 treatment.
That September event, part of which occurred in the White House Rose Garden, was crowded with guests, many of whom did not wear masks.