WASHINGTON — Sandra Day O’Connor, the first woman on the Supreme Court, announced today that she has the beginning stages of dementia, “probably Alzheimer’s disease.”

O’Connor made the announcement in a letter. The 88-year-old said her diagnosis was made some time ago and that as her condition has progressed she is “no longer able to participate in public life.”

“While the final chapter of my life with dementia may be trying, nothing has diminished my gratitude and deep appreciation for the countless blessings in my life.

“How fortunate I feel to be an American and to have been presented with the remarkable opportunities available to the citizens of our country. As a young cowgirl from the Arizona desert, I never could have imagined that one day I would become the first woman justice on the U.S. Supreme Court,” she wrote.

O’Connor was nominated by President Ronald Reagan and took her seat on the court in 1981.

She was 75 when she announced her retirement from the court in 2005. It was a decision influenced by the decline in the health of her husband, John O’Connor III, who himself had been diagnosed with Alzheimer’s disease.

O’Connor was a state court judge before being unanimously confirmed to the Supreme Court at 51. She had graduated third in her class from Stanford Law School and was the first woman to lead the Arizona state Senate.

On the Supreme Court, her votes were key in cases about abortion, affirmative action and campaign finance as well as the Bush v. Gore decision effectively settling the 2000 election in George W. Bush’s favor.

O’Connor’s announcement of her diagnosis came a day after a story by The Associated Press that she had stepped back from public life.

Her son Jay O’Connor said in that story that his mother had begun to have challenges with her short term memory. He also said that hip issues have meant she now primarily uses a wheelchair and stays close to her home in Phoenix.

For her part, O’Connor wasn’t always delighted with the court’s more conservative direction after she left. Asked at a 2009 event how she felt about the court retreating from or undoing rulings she was instrumental in shaping, she responded: “What would you feel? I’d be a little bit disappointed. If you think you’ve been helpful, and then it’s dismantled, you think, ‘Oh, dear.’ But life goes on. It’s not always positive.”

After the court’s 2010 Citizens United ruling allowing corporations to spend freely on elections for Congress and president, she told an audience: “Gosh, I step away for a couple of years and there’s no telling what’s going to happen.” Still, that year she told an interviewer that she didn’t “regret for one minute” retiring when she did.

O’Connor found other ways to make a mark off the court. In 2009, the same year her husband died, she founded the group iCivics, which promotes civic education in schools through free, educational online games. O’Connor has called it “the most important work I’ve ever done.” Last year, the group’s 19 games were played by 5 million students.

Even as she was championing iCivics, O’Connor was working on other projects. She wrote a children’s book and a book about the history of the court. She served as a visiting appeals court judge, participating in more than 175 cases on appeals courts nationwide. And she campaigned to persuade states that judges should be appointed, not elected, to preserve judicial independence.

One of the last times O’Connor made public comments was in 2016, after Justice Antonin G. Scalia’s death. Interviewed by an Arizona television station, O’Connor was asked what she thought about Republican senators’ argument that the conservative justice’s seat should be filled not by President Barack Obama but his successor because the vacancy happened in a presidential election year. She said she disagreed.

“I think we need somebody there, now, to do the job, and let’s get on with it,” she said, a recommendation Republicans didn’t heed, holding the seat open until President Donald Trump could choose Scalia’s successor, Neil M. Gorsuch.

Though O’Connor has stepped back from public life, the court’s other retired justices have varying degrees of public presence. David H. Souter, 79, lives in New Hampshire but rarely speaks publicly. Kennedy, 82, has already started making post-retirement appearances. Florida resident John Paul Stevens is still making appearances at 98.

Three women now serve on the Supreme Court, a development O’Connor approved of.

“It’s all right to be the first to do something, but I didn’t want to be the last woman on the Supreme Court,” she said in 2012.