The Washington Post Co. announced yesterday that it has sold the Trenton Times to Washington financier Joe L. Allbritton, who immediately assumed control of the New Jersey daily and Sunday newspaper.

Spokesmen for the Post Co. and Allbritton Communications Co. declined to disclose the purchase price, but informed sources estimated that Allbritton paid about $10 million or $11 million for The Times, which has been losing money for three years. The Post Co. acquired the newspaper in 1974 for $16 million in cash and notes.

Allbritton Communications, based in Washington, became the largest newspaper group owner in New Jersey with yesterday's purchase. The company owns dailies in Paterson and Union City and will move its newspaper division headquarters to Trenton. Other properties of Allbritton's firm include WJLA-TV (Channel 7) here, newspapers in Massachusetts and broadcast stations in Virginia, South Carolina and Kansas.

A former owner of the defunct Washington Star, Allbritton also is board chairman and the controlling stockholder of Riggs National Corp., Washington's largest financial institution.

George Beveridge, a spokesman for Allbritton, said yesterday that it is "too early to have made any decisions" about future employment for the 325 employes of Trenton Times Corp., but other sources said Allbritton plans a substantial cutback in the paper's 80-person editorial staff.

Beveridge and Edward L. Hoffman, publisher of the rival Trentonian in New Jersey's capital city, both dismissed suggestions that the two publishing companies might seek an agreement on a joint publishing plan. "We're in this situation to win . . . we want to make it a strong local newspaper," Beveridge said.

Martin Cohen, vice president for finance at the Post Co., said that losses at Trenton over the past three years "have not been material relative to our financial position" although he declined to provide specific loss figures or the impact on Post Co. earnings. Third-quarter results for the communications firm will be released next week, he added.

Asked why the Post Co. decided to sell the Trenton newspaper, Cohen said Allbritton's firm is "well-equipped" to serve the New Jersey market because of its presence there already.

Post Co. Chairman Katharine Graham said in a prepared statement: "While we are sad to be leaving, we are convinced that Allbritton Communications . . . is committed to publishing a fine newspaper which will be an asset to Trenton."

The Post Co.'s other newspaper properties are The Post here and the Everett (Wash.) Herald.

Trenton Times president and publisher Edward Padilla will return to the Post Co. as vice president of its newspaper division. Times executive editor Larry Kramer also will return to The Post, according to yesterday's announcement. Kramer, a former financial reporter at The Post, told his staff of the shift in ownership yesterday afternoon and said later that Allbritton has made a "strong statewide commitment . . . they have a very definite game plan" to make the paper profitable.

Allbritton announced that W. Dean Singleton, executive vice president of his communications firm, would serve additionally as president and chief executive in Trenton. A New York Daily News editor, Thomas Curran, succeeded Kramer as editor. All other department heads remained as of last night.

Newspaper industry analyst John Morton of the securities firm of Lynch, Jones & Ryan here said Allbritton's purchase "baffles me . . . the prospect for an aging and weak newspaper in a fading, industrial market is not good . . . the Trentonian is a very successful competitor and The Times has steadily lost ground no matter what anybody has done, including The Washington Post."

But Allbritton said in a statement that purchase of The Times "is a major step in our company's expansion of its publishing interests."

Industry sources said Allbritton may seek to make Trenton's Sunday paper more of a statewide publication. His other two morning dailies, the Hudson Dispatch in Union City (circulation 41,000) and the Paterson News (51,000), do not have Sunday editions.

Although nominally an afternoon paper, The Times has been moving rapidly into morning competition with the Trentonian, a newspaper that concentrates strongly on local news. Beveridge said about 40 percent of The Times' daily circulation is now in the morning and "we will want to move in that direction on a gradual basis."

Rival publisher Hoffman welcomed Allbritton, as he had the Post Co. in the 1970s. The Trentonian is owned by the Ingersoll Newspapers group, and its daily circulation is approximately even with The Times for the first time, according to the latest circulation figures, which show both papers in the 65,000-68,000 range. The Times continues to have a larger readership on Sundays, with circulation of about 80,000, or 20,000 more than the Trentonian.

Beveridge said The Times has about 62 percent of the market's advertising linage and 70 percent of the ad dollars, but Hoffman said "it appears to us" that the Trentonian is splitting the ad market about 50-50. The two newspapers have long been engaged in a bitter competitive battle, complete with differing assessments of their market share.

Both companies declined to state which party had initiated talks on selling the Trenton paper, but one spokesman said negotiations had not been going on "very long." The 100-year old Times was sold to the Post Co. in 1974 by the Kerney family. At the time, daily circulation was listed at about 75,000 and Sunday circulation at 103,000.