Sales totaled $935.4 million in the quarter covering the holiday season, down 20.1 percent from $1.17 billion in the year-ago period. Analysts, on average, looked for sales of $1.12 billion, according to Thomson Reuters I/B/E/S.
Sales at stores open at least a year fell 19 percent in the fourth quarter on weak customer traffic.
The grim results were not entirely unexpected, considering the overall weakness in the consumer electronics industry during the holidays, but many on Wall Street took a grim view of the company's prospects.
The "results were much worse than we anticipated, and cast serious doubt on RadioShack's long-term viability in our opinion," BB&T Capital Markets analyst Anthony Chukumba said.
The report also highlighted the mammoth task facing Chief Executive Officer Joe Magnacca, who took the helm in February 2013.
Magnacca, a restructuring expert credited with revamping Duane Reade drugstores before Walgreen bought the chain, said "the RadioShack turnaround will take time."
David Tawil, the co-founder of hedge fund Maglan Capital, said the move to shrink its store base was not a permanent solution to its problems.
"It should buy them time," said Tawil, a former bankruptcy attorney and distressed corporate workout specialist. "I don't think they have a place in the market." Maglan does not own RadioShack shares.
Its market share has fallen about 20 percent since 2010, according to data from Euromonitor International.
Tawil said he is concerned about its cash flow levels. "They can continue to operate but they are in a cash-flow negative spiral and that doesn't usually reverse itself," Tawil said.
(Read more: Eh, don't put too much stock in retail earnings)
The retailer, which secured new loans heading into the holidays, ended the fourth quarter with total liquidity of $554.3 million, including $179.8 million in cash and cash equivalents and $374.5 million in available credit.
At Dec. 31, its debt totaled $614 million and matured between 2018 and 2019.
RadioShack peers Best Buy and Hhgregg also reported weaker-than-expected sales in what turned out to be the most heavily discounted holiday shopping season since the recession.
Magnacca blamed the latest sales weakness on poor shopper traffic, intense discounting, tepid mobile phone demand and operational problems such as poor inventory management.
"Mr. Magnacca and the new team deserve a lot of credit for the changes they are making, but it seems harder and harder for them to overcome the more significant obstacles," said Janney Capital Markets analyst David Strasser. He worried that wireless industry weakness could stall its turnaround.
RadioShack has been working with bankers from Peter J Solomon Co to boost liquidity, and with AlixPartners on improving operations. Last October, it obtained $835 million in financing commitments from a consortium of lenders led by GE Capital; CIT Corporate Finance; RBS Citizens NA and Salus Capital Partners.
Under Magnacca, RadioShack has changed its logo, reduced store clutter and improved displays. It has also been moving some products from stores to its website, and carrying more private-label goods with higher margins.
The retailer recently named a new merchandising chief, global sourcing chief and chief financial officer. But some analysts say the efforts are too little, too late.
The stock slid 13.6 percent to $2.35 on the New York Stock Exchange, but fell as low as $2.19. (Click here for the latest quote.)