If Trump Plaza closes, Atlantic City could lose a third of its casinos and a quarter of its casino workforce in less than nine months. The Atlantic Club closed in January, the Showboat is closing next month, and the Revel might do likewise if a buyer can't be found in bankruptcy court.
Trump Entertainment Resorts told the AP that its managers and board of directors "have been reviewing alternatives for the property. Although this review has not been completed and no final decision has been made, the company expects that it will terminate the operations of Trump Plaza Hotel and Casino on or shortly after September 16, 2014."
Read More Atlantic City's Showboat Casino plans to close
A source with direct knowledge of the situation who was not authorized to speak to the media told the AP that the company has hired a search firm to solicit buyers for Trump Plaza, an effort that remains ongoing. So far, no buyer has emerged.
The company did not indicate what might become of the building after it is closed.
Trump Plaza, which cost $210 million to build, opened in May 1984 as one of Donald Trump's pet projects. The real estate mogul has since limited his dealings in Atlantic City to a 10 percent stake in Trump Entertainment Resorts and does not control its day-to-day operations.
The news is the latest in a cascade of setbacks for Atlantic City's gambling market, which, until a few years ago, was the second-largest in the nation after Nevada; Pennsylvania has now taken over that spot. Analysts have long said that the casino market in Atlantic City, and in the Northeastern United States, has been oversaturated, and that some casinos need to close to ensure the survival of others.
Read MoreAtlantic City's Revel casino files for bankruptcy
That's what's behind the decision by Caesars Entertainment, the world's largest casino company, to shutter the still-profitable Showboat next month. CEO Gary Loveman said Atlantic City has too many casinos. The company will still operate three Atlantic City casinos.
Caesars also joined with Tropicana Entertainment to buy the Atlantic Club from bankruptcy court last December, divide its assets and shut it down.
On Jan. 1, Atlantic City had 12 casinos. By the end of September, it could have eight.