Chief Justice John Roberts administered the oath to Kagan in a brief private ceremony at the court. Kagan, joined by family and friends, pledged to faithfully and impartially uphold the law.
Afterward, she smiled broadly as a crowd of onlookers stood and applauded. "We look forward to serving with you," Roberts said.
Kagan, a former Harvard Law School dean who most recently was solicitor general, was President Barack Obama's choice to succeed retired Justice John Paul Stevens. Republicans criticized her as a political liberal, before the Senate confirmed her this past week on a vote of 63-37.
She was sworn in twice Saturday by Roberts -- reciting one oath as prescribed by the Constitution during a ceremony in a conference room at the court with only her family present. Kagan then recited a second oath, taken by judges, with her family and friends and reporters present.
Kagan won't be formally installed as a justice until Oct. 1 in a courtroom ceremony at the start of the court's new term. But after the oaths taken on Saturday, she will be able to begin assuming her duties as a justice, which include reviewing cases and emergency appeals filed to the Supreme Court.
Kagan, 50, joins Justices Ruth Bader Ginsburg and Sonia Sotomayor on the nine-member court, which often divides 5-4 on high-profile cases such as gun rights, discrimination and campaign finance. The first woman in the court's history, Justice Sandra Day O'Connor, retired in 2005.
Kagan isn't expected to alter the ideological balance of the court, where Stevens was considered a leader of the liberals.