Ms. Abramson has been one of Mr. Keller's two top deputies since 2003, serving at his side as he steered The Times through a period of journalistic distinction and economic distress. Mr. Keller said that with the paper's finances now on surer footing, he felt at ease handing the reins to Ms. Abramson.
The move was accompanied by another shift in senior management. Dean Baquet, the Washington bureau chief and former editor of The Los Angeles Times, will become the managing editor for news.
Arthur Sulzberger Jr., the paper's publisher and the chairman of The New York Times Company, thanked Mr. Keller, calling him a "truly valued partner" in a speech Thursday morning in the Times newsroom, where the staff stood shoulder to shoulder to hear the publisher announce the first changeover in the top editing jobs in eight years.
Turning toward Ms. Abramson, who will become the first woman to be editor of the paper in its 160-year history, Mr. Sulzberger said, "Jill, Bill's decision to step down may be bittersweet. But the thought of you as our next executive editor gives me and gives all of us great comfort and great confidence."
The appointments are effective Sept. 6. John M. Geddes, 59, will continue in his role as managing editor for news operations.
Over the course of Mr. Keller's tenure, the paper won 18 Pulitzer Prizes and expanded its online audience to some 50 million readers worldwide. But the economic downturn and the drift of readers and advertisers to the Web also forced the paper to lay off members of the news staff and tighten budgets considerably.
"A couple of years ago, everybody was wringing their hands about doomsday for the news business," Mr. Keller said to the staff, his voice emotional at times. "People talked, some of them rather smugly, about even The New York Times not being long for this world. And now you look around, and we are economically sturdy. We are rich in talent. We are growing."
Mr. Keller will continue to write for The Times Magazine and as a columnist for the new Sunday opinion section, which will make its debut this month. Mr. Sulzberger said he accepted Mr. Keller's resignation "with mixed emotions," adding that the decision to leave was entirely Mr. Keller's.
Mr. Keller, 62, is still a few years shy of the paper's mandatory retirement age for senior executives, but he held the top job for roughly the same period of time as Max Frankel and Joseph Lelyveld, two of the editors who preceded him. Mr. Frankel and Mr. Lelyveld returned to the newsroom for the announcement.
Mr. Keller had asked Ms. Abramson to be his managing editor in 2003 as he assembled a team that he hoped would restore confidence in the paper after the Jayson Blair plagiarism scandal. Ms. Abramson had been part of a group of editors who clashed with Howell Raines, the executive editor who was forced out after Mr. Blair's fraud was discovered.
Ms. Abramson, 57, said being named executive editor was "the honor of my life" and like "ascending to Valhalla" for someone who read The Times as a young girl growing up in New York. "We are held together by our passion for our work, our friendship and our deep belief in the mission and indispensability of The Times," she said. "I look forward to working with all of you to seize our future. In this thrilling and challenging transition, we will cross to safety together."
The selection of Ms. Abramson is something of a departure for The Times, an institution that has historically chosen executive editors who ascended the ranks through postings in overseas bureaus and managing desks like Foreign or Metro.

Ms. Abramson came to The Times in 1997 from The Wall Street Journal, where she was a deputy bureau chief and an investigative reporter for nine years. She rose quickly at The Times, becoming Washington editor in 1999 and then bureau chief in 2000. She stepped aside temporarily from her day-to-day duties as managing editor last year to help run The Times's online operations, a move she asked to make so she could develop fuller, firsthand experience with the integration of the digital and print staffs.

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