Brendan McDermid/Reuters

William A. Ackman sold his roughly 18 percent stake in J.C. Penney at $12.90 a share on Monday, severing all ties to the retailer two weeks after he resigned from the board. At that price, his stake was valued at $504 million, saddling him with a loss of about $473 million.

The hedge fund manager William A. Ackman moved on Monday to sell his roughly 18 percent stake in J. C. Penney, nearly two weeks after he resigned from the board amid an unusual public battle with his fellow directors.

Penney filed a prospectus with regulators giving notice that Mr. Ackman’s firm, Pershing Square Capital Management, planned to sell its 39.1 million shares. The retailer, which will not receive any proceeds from the sale, did not list an expected selling price.

Related Links Document: Penney prospectus

Shares in Penney closed at $13.35 on Monday, valuing the stake at about $522 million. Pershing first began buying stock in the company three years ago, becoming the biggest shareholder in the process, and paid on average about $25 a share for its position. If its holdings were sold around Monday’s closing price, the hedge fund would face a potential loss of about $456 million. (Mr. Ackman’s firm also faces more losses on derivatives it owns that are tied to Penney’s stock.)

A spokeswoman for Mr. Ackman declined to comment beyond the prospectus.

The stock sale came six days after the retailer announced its latest quarterly earnings. Under an agreement with Penney’s board, Mr. Ackman could not begin disposing of his stake until then because he still possessed some material information about the company’s finances.

Mark Lennihan/Associated Press

By selling his stake, Mr. Ackman will wash his hands of a drawn-out and ultimately disappointing investment in Penney. He first emerged as a big investor three years ago, believing that the retailer could be turned around with new management. To that end, he enlisted Ron Johnson, the celebrated architect of Apple’s retail strategy.

But Mr. Johnson’s tenure proved to be disastrous, with numerous initiatives — eliminating discount sales, a costly renovation of Penney’s stores — serving only to drive away existing customers while failing to bring in new ones. Earlier this year, the board fired Mr. Johnson and brought in his predecessor, Myron E. Ullman III, on an interim basis.

Earlier this summer, Mr. Ackman again grew anxious that Penney was on the wrong track, fretting about executive appointments by Mr. Ullman that the hedge fund manager believed did not follow established procedures. Concerned that the board was becoming fractured, he later publicly called for the replacement of its chairman, Thomas J. Engibous, with Allen I. Questrom, a former Penney chief.

Brendan McDermid/Reuters

The two sides negotiated a truce: Mr. Ackman would step down, while Ronald W. Tysoe, a longtime retailing executive, would join the board, with another new director to follow soon.

Yet as Mr. Ackman sells off his position, other investors appear to be betting that Penney is on the verge of a turnaround. The investment firm founded by George Soros now owns a roughly 9 percent stake, while Perry Capital recently announced a 7.3 percent stake.

And Hayman Capital, a firm run by Kyle Bass, reportedly has both bought up shares and made a bullish bet on the retailer’s debt, according to news reports.