NEW YORK — J.C. Penney executives may be confident in the department-store chain’s everyday low pricing strategy, but its investors are panicking.

The company’s stock fell more than 13 percent on Monday — the biggest percentage decline by far among big companies in the S&P 500 for the day. Penney stock lost nearly $3 on Monday to close at just under $18, its lowest price since March 2009 when the nation was in a recession.

The drop follows Standard & Poor’s Ratings move to lower Penney’s credit rating deeper into junk status on Friday. And that came on the same day that the company reported its third consecutive quarter of big losses and sales declines since it decided earlier this year to get rid of hundreds of coupons and sales each year in favor of predictable low prices every day.

It’s the latest sign that Wall Street isn’t any happier with Penney’s pricing than Main Street is. Investors had pushed Penney stock up 24 percent to about $43 after the company announced the pricing plan in late January. But customers haven’t warmed to Penney’s pricing, and investors have grown cold on the stock. With Monday’s drop, Penney’s stock has lost nearly half of its value this year.

Penney, which announced its plans for the holiday shopping season on Monday, did not immediately return calls seeking comment about its stock price.