A resurgent Kmart, home of the blue light special, is buying the once-dominant Sears department store chain in a surprising $11 billion gamble it is counting on to help both better compete with Wal-Mart and other big-box retailers.

Led by Kmart Holding Corporation’s chairman, Edward Lampert, the new Sears Holdings Corporation would be the nation’s third-largest retailer. Both chains would survive, but several hundred stand-alone Kmarts throughout the country are expected to be transformed into Sears stores. The goal: A quick kick-start to sales away from Sears’s traditional base of shopping malls.

In announcing the deal yesterday, Mr. Lampert and Sears’s chairman and CEO, Alan Lacy, promised up to $500 million a year in savings within three years from store conversions, back-office job cuts, more efficient buying of goods, and possible store closings.

Shares of both Kmart and Sears, Roebuck and Company surged on the news, but some analysts are skeptical that it amounts to a home run.

“Both have been broken in some sense,” said the president and chief executive of Merchant Forecast, a New York-based independent research company, Dan Hess. “Kmart has to learn to survive in a Wal-Mart world and Sears needs to learn to survive in a world of Home Depot and Lowe’s.”

Mr. Lampert, 42, was an assistant to Robert Rubin at Goldman Sachs & Company before leaving to form a hedge fund at the age of 25. He orchestrated the deal and will lead a new board that will be dominated by Kmart directors.

“We need to have a low-cost structure to compete with bigger retailers,” said Mr. Lampert, whose Greenwich, Conn.-based investment firm controls Kmart and is Sears’s largest individual shareholder, with a 15.8% stake.

For Sears, the merger allows the company to move more quickly to where it believes its strongest base of customers is located. “Off-mall is where we need to move very aggressively,” said Mr. Lacy, who will become vice chairman and chief executive of Sears Holding.

Mr. Lacy said he and Mr. Lampert have known each other for four years. The idea for a combined company first arose when they were in talks about Sears’s purchase of 50 Kmart stores earlier this year, he said.

The new company is expected to have $55 billion in annual revenues and 3,500 outlets. That will mean it will trail only Wal-Mart Stores Inc. and Home Depot Inc. among the biggest American retailers.

It will be headquartered in the northwestern Chicago suburb of Hoffman Estates, where Sears has its headquarters, but will maintain a “significant presence” in Troy, Mich., where Kmart is based.

The deal marks a remarkable comeback for Kmart, which filed for Chapter 11 bankruptcy protection in early 2002, leading to the closing of about 600 stores, termination of 57,000 Kmart employees, and cancellation of company stock.

Mr. Lampert gained control of Kmart when the retailer emerged from bankruptcy in May 2003 through the conversion of his debt holdings into equity. In March, Kmart posted its first profitable quarter in three years.

While same-store sales have continued to decline, Mr. Lampert has maximized cash flow in part by selling off some of the stores to Sears and Home Depot.

Kmart yesterday announced it earned $553 million, or $5.45 a share, in the third quarter ended October 27, compared with a loss of $23 million, or 26 cents a share, for the same period a year ago. Its stock price has risen more than sevenfold from $15 a share when it emerged from bankruptcy.

Sears’s roots date to the late 1800s, when it offered merchandise by mail order to farmers. It opened its first retail store in 1925 and eventually became the nation’s biggest department store operator.

Mired in a retail slump, Sears had long fallen out of favor on Wall Street after losing ground to competitors and enduring sluggish sales for years. The company last fall introduced its Sears Grand stores, which offer grocery and convenience items in addition to traditional Sears fare such as clothing, home appliances, and tools. The concept delivered promising results for the retailer at its first three stores in metropolitan Salt Lake City and Las Vegas and in the Chicago suburb of Gurnee.

Mr. Lampert said the goal for the combined company is to achieve a 10% operating profit margin, a level that’s generated by such retailers as Gap Inc. and Target Inc. He noted that in the meantime the financial operations will be “lumpy” as they digest the two companies.

A key part of increasing productivity at the stores will be in the cross-selling of the brands, though company officials declined to be specific on which they would overlap. Besides Craftsman tools and Kenmore appliances, Sears’s exclusive brands include Lands’ End cloth ing. Kmart’s brands include Martha Stewart, Jaclyn Smith, and Joe Boxer. Mr. Lampert said Sears could also benefit from Kmart’s expertise in its pharmaceutical department and health and beauty products.

Mr. Lampert said it is unlikely that any Sears stores would be converted to Kmarts, and that store closings are a possibility. “I think we’ll probably end up over time opening more stores than we close, but obviously if we don’t operate the stores well, it might be the other way around,” he said.

He also would not provide any details on possible layoffs, except to say, “There will be some head count changes that come out of this.”

Sears shares soared $7.79, or more than 17%, to $52.99 in afternoon trading on the New York Stock Exchange, and Kmart shares climbed $7.78, or close to 8%, to $109 on the Nasdaq Stock Market.

Shares of Martha Stewart Living Omnimedia Inc. rose 6.26% to $18.49 on the belief among investors that the deal could bring a larger-scale merchandising agreement with Sears. Currently, Martha Stewart Everyday brand is sold exclusively at Kmart in America and at Sears Canada.

Under yesterday’s agreement, which was unanimously approved by both companies’ boards of directors, Kmart shareholders would receive one share of new Sears Holdings stock for each Kmart share. Sears shareholders can choose $50 in cash or half a share of Sears Holdings stock. That portion of the deal values Sears shares at $11 billion, a 10.6% premium over its value at Tuesday’s close.

The merger, expected to close by the end of March, is subject to approval by Kmart and Sears shareholders, regulatory approvals, and customary closing conditions.

Sears Holding also created the office of the chairmanship, which consists of Messrs. Lampert and Lacy, and Aylwin Lewis, who was named president of Sears Holding Corporation and CEO of Sears Retail. Last month, Mr. Lewis, formerly an executive at restaurant operator Yum Brands Inc., was named chief executive and president of Kmart.