Sotomayor, 55, flanked by her mother and brother, was administered the oath by Justice John Roberts. A 17-year veteran of the federal court system, she pledged to uphold the Constitution and to administer impartial justice.
With her left hand placed on a Bible, the former prosecutor who rose from poverty in New York City, also pledged to ''do equal right to the poor and to the rich.''
About 60 people, including other family members and representatives of the Obama administration, attended the ceremony.
Sotomayor replaces retired Justice David Souter. She was handpicked in May for the post by President Obama and was confirmed August 6 in a Senate vote of 68 to 31.
Sotomayor assumed the bench in a two oath processes: The first took place minutes before the second in a private ceremony in a room reserved for the justices’ private conferences.
Roberts said that once the oaths were done, Sotomayor could ''begin work as associate justice without delay.''
In her first Supreme Court hearing, she will sit among the other eight justices on September 9 for arguments in a case involving campaign finances.
The justices, who are paid just over $200,000 annually, typically hear between 75 and 100 cases each year.