Ackman bought 5 million shares in Sears, a spokeswoman for his New York-based Pershing Square Capital Management said, confirming a report by StreetInsider.com. He disclosed the stake in a speech at a Dallas charity event earlier this week where he said he was "thrilled" with Chairman Edward Lampert's efforts at Sears and was "looking forward to working with him," according to the Wall Street newsletter.

While Ackman's large stakes and agitating have succeeded in forcing real estate sales or other actions at McDonald's Corp., Wendy's International Inc. and Target Corp. in the past, his impact at Sears could be limited since Lampert, who also runs a hedge fund, maintains a 46 percent stake and full control.

But the purchase highlighted Sears' recent stock rally after a lengthy decline.

Sears shares jumped $7.84, or 5.5 percent, to close at $149.24 after sinking as low as $129.75 on Tuesday, the day of his speech. The stock had declined nearly 37 percent from its all-time high of $195.18 on April 17 to its 52-week low of $123.39 last week.

Some analysts speculated that Ackman could be eyeing Sears Canada, an asset where he has clashed with Lampert before. Sears Canada shareholders rejected a $792 million takeover bid by Sears last November after Ackman, a major shareholder of the Canadian unit, argued it was worth much more.

Ackman previously owned more than 1.5 million shares of Sears as recently as March 2006, suggesting that he was attracted back to the stock this fall by the fallen share price.