CHICAGO—For more than two decades, Oprah Winfrey has been the inspirational, change-your-life champion who reigned over daytime television much like Johnny Carson once ruled late night.

Now she’s ready to say goodbye, leaving a huge void for broadcast TV even as she raises the possibility of more Oprah than ever when she starts her own cable network.

Winfrey told viewers Friday that she will dim the lights on “The Oprah Winfrey Show” at the close of its 25th season in late 2011.

“I love this show. This show has been my life. And I love it enough to know when it’s time to say goodbye,” she said, holding back tears. “Twenty-five years feels right in my bones, and it feels right in my spirit. It’s the perfect number, the exact right time.”

Winfrey talked about being nervous when the program began in 1986 and thanked audiences who had invited her into their homes and lives over the past two decades.

“I certainly never could have imagined the yellow brick road of blessings that have led me to this moment,” she said.

For the hundreds of network affiliates who depended on Winfrey to deliver millions of viewers every day, Friday’s announcement starts an 18- month clock to find a way to fill the space left behind after the end of the most successful daytime talk show in television history.

Winfrey’s show “is one of daytime television’s very foundations,” said Larry Gerbrandt, an analyst for the firm Media Valuation Partners in Los Angeles. “You could, and stations did, build their schedules around her. They gave it the best time period, leading into their news, and used it to promote other shows.”

Winfrey cautioned viewers that they would hear “a lot of speculation in the press about why I am making this decision,” warning them not to listen to the “conjecture.” But she offered no specifics about her plans for the future, except to say that she intended to produce the best possible shows during the final two years.

“I just wanted to say whether you’ve been here with me from the beginning or you came on board last week, I want you all to know that my relationship with you is one that I hold very dear,” she said. “Your trust in me, the sharing of your precious time every day with me has brought me the greatest joy I have ever known.”

It has also brought her a fortune estimated at $2.7 billion. As a newcomer, she chipped away at the dominance of Phil Donahue. She flirted with a tabloid format for a time but gradually reinvented her show to focus on themes of inspiration, hope and the power of positive thinking.

The show has a breadth that no other has been able to match. A serious hour on domestic abuse could be followed the next day by a rollicking party with the Black Eyed Peas.

But even Winfrey was not immune to the dips in ratings that have plagued broadcasters as viewers flock to specialty programming on cable. Her average audience — easily the largest of daytime talk shows — fell from 12.6 million in 1991-92 to 6.2 million in 2008-2009. This season, the show is averaging 7.2 million viewers a day.

Winfrey, 55, is widely expected to start up a new talk show on OWN: The Oprah Winfrey Network, a joint venture with Discovery Communications Inc. that was first announced last year.