President Biden formally announced the retirement of Justice Stephen Breyer on Thursday, a step that kicks off a weekslong political clash on Capitol Hill as Mr. Biden seeks to put his imprint on the Supreme Court.

Mr. Biden said he would name a successor by the end of February after a deliberate process, but he declined to answer questions about that process, saying that the day should be focused on Justice Breyer’s career.

Speaking with the justice by his side from the Roosevelt Room, Mr. Biden hailed the justice’s four decades on the federal bench, including almost 28 years on the Supreme Court.

Mr. Biden called it a “bittersweet day” and praised Justice Breyer for his “remarkable career of public service and his cleareyed commitment to making our laws work for the people.”

“I think he’s a model public servant in a time of division in this country,” Mr. Biden said.

In a letter to the president made public just after noon, Justice Breyer confirmed his retirement, calling it “a great honor” to have participated in “the effort to maintain our Constitution and the rule of law.”

In brief remarks after the president spoke, Justice Breyer recalled what he often told students who ask about his job. He said he marveled at the diversity of opinions that came before the court and how the American “experiment” managed to continue.

“It’s every race, it's every religion and it’s every point of view possible,” he said, attributing that quotation to his mother. “It’s a kind of miracle. People that are so different in what they think, and yet they’ve decided to help solve their major differences under law.”

In his letter, Justice Bryer made it clear that he would continue to serve on the court until the end of the term and would not leave before a successor is confirmed by the Senate.

“I intend this decision to take effect when the Court rises for the summer recess this year (typically late June or early July), assuming that by then my successor has been nominated and confirmed,” Mr. Breyer wrote.

That timeline will give Mr. Biden several months to find a successor for Justice Breyer while being confident that he will stay on the court until the new justice is confirmed by the Senate.

In the days ahead, Mr. Biden will be free to make good on the promise he made as a presidential candidate in 2020. Fighting for the Democratic nomination, he pledged to be the first president to select a Black woman for a life appointment to the court.

That decision of whom to nominate as a replacement for Justice Breyer, a liberal jurist, will not affect the ideological balance on the court, where conservatives hold a 6-to-3 majority. But Mr. Biden — who has said repeatedly that he views efforts to promote diversity as a big part of his legacy — is poised to make a historic and long-lasting imprint on what the court looks like.

In his remarks on Thursday, Mr. Biden said he would look broadly for advice from outside legal experts, senators and others. He made a point of saying that he would look to Vice President Kamala Harris, whom he called an “exceptional lawyer.” She is a former attorney general of California who served on the Judiciary Committee when she was in the Senate.

“I will listen carefully to all the advice I’m given, he said.

Still, speculation has already focused on three Black jurists seen as the most likely candidates. They are Ketanji Brown Jackson, a 51-year-old judge on the U.S. Court of Appeals for the District of Columbia Circuit; Leondra R. Kruger, a 45-year-old justice on the California Supreme Court; and J. Michelle Childs, 55, a Federal District Court judge in South Carolina whom Mr. Biden recently nominated for a judgeship on a federal appeals court.

The president could still pick someone else, and he is not required to elevate someone who is already a judge, though that is by far the most common route to the Supreme Court. Some of Mr. Biden’s predecessors have picked politicians, lawyers or law professors.

But the president is not expected to stray from his pledge to ensure that his pick is a Black woman, and he emphasized that pledge on Thursday.

In his letter, Justice Breyer wrote that he appreciated the privilege of serving on the court for almost 28 years, saying “I have found the work challenging and meaningful. My relations with each of my colleagues have been warm and friendly.”