Justice Souter was appointed to the Supreme Court in 1990
US Supreme Court Justice David Souter is to step down at the end of the court's current term, President Barack Obama has confirmed.
The 69-year-old is one of four liberal-leaning judges on the bench of nine.
Mr Obama said he hoped to appoint a replacement for him by the time the court's next session begins in October.
Analysts say his retirement is unlikely to affect the balance of the court, which also includes four conservative judges and a centrist.
Justice Souter was appointed to America's top court in 1990 by then President George H W Bush.
President Obama took the unusual step of interrupting White House press secretary Robert Gibbs' daily briefing to announce that he had just spoken to Justice Souter.
Confirming the judge's plans to step down, Mr Obama said he was "incredibly grateful for his dedicated service".
He said Justice Souter had shown what it meant to be a fair-minded and independent judge and that he was looking for someone who was "dedicated to the rule of law" to replace him.
"As I make this decision, I intend to consult with members of both parties, across the political spectrum," Mr Obama said.
Some expect him to choose a woman - who would be only the third to serve on the Supreme Court.
The BBC's Jamie Coomarasamy in Washington says that Mr Obama has been given an early chance to make one of the most consequential decisions that falls to a US president - a lifetime appointment to the country's highest court.
Every Supreme Court nomination is a highly politicised affair and this one will be no different, our correspondent says.
Michael Steele, chairman of the Republican National Committee (RNC), urged Mr Obama in a statement to "take his time and search for a nominee whose legal views are consistent with and reflective of mainstream America".
Mr Steele said: "He should not use this as an opportunity to impose his liberal legacy on America or give political payback to the far left of his party by nominating someone who makes judicial decisions through an ideological rather than legal lens."