Joni Evans, president of Simon and Schuster's trade books division and one of the most prominent figures in New York publishing, is leaving the company to launch her own imprint at Random House.

Evans' sudden defection to Simon and Schuster's chief rival marks the latest in a series of expansionary moves and high-profile hirings by Random House and its energetic chairman, Robert L. Bernstein.

Since the first of the year, Bernstein has asserted his company's international intentions by annexing a consortium of three respected British book imprints -- Chatto & Windus, The Bodley Head and Jonathan Cape, Ltd. -- and by hiring Sonny Mehta as president and editor in chief of Alfred A. Knopf, the most literary of the Random House imprints, to succeed Robert Gottlieb when he became editor of The New Yorker.

Last month alone, Random House made publishing news by acquiring the independent publisher Schocken Books (merging it with its existing Pantheon imprint) and by luring the respected editor Elizabeth Sifton away from Viking Press (installing her as executive editor of Knopf).

Evans, who started her career in publishing as a manuscript reader at McCall's 24 years ago, became president of Simon and Schuster in 1985. The imprint she founded there six years before, Linden Press, came to be considered the literary jewel of the publishing conglomerate and sped her ascent to the presidency of the trade books division.

Evans married Simon and Schuster Chief Executive Officer Richard E. Snyder in 1978. Their marriage collapsed last year, and her departure consequently had been predicted in New York publishing circles.

Evans' last day at Simon and Schuster will be Sept. 4, and though Random House was vague about the effective date of her appointment, "knowing me, I'll be starting September 7," Evans said yesterday.

She said the editorial offices of her new imprint, as yet unnamed, would be housed apart from the East 50th Street office tower where Random House is headquartered. "It was important to me to distinguish my imprint from the many, many imprints that are there," she said. "There will be a physical distinction as well as an editorial distinction."

Bernstein was reluctant to speculate about plans for the Evans imprint. "There's no five-year plan in publishing, just straight muddling through," he said. "My theory is when you find people who are very good, you get them and cling to them."

Evans said she hoped to publish "the kinds of books I've always published -- fiction of high quality or fiction that's highly commercial, and nonfiction that's terribly contemporary." Given the typical gestation of book properties, Evans' books are not likely to appear before 1989.

Among the authors Evans published in her 13 years at Simon and Schuster are John Gregory Dunne, Mario Puzo, Robert Coover, Candice Bergen, Andrew Tobias, Ann Beattie, Jeffrey Archer and Anita Brookner.

A spokeswoman for Simon and Schuster said there were "no plans at this time" to replace Evans as president of the trade division. At least for the meantime, her responsibilities will be assumed by Michael Korda, vice president and editor in chief, and Alice Mayhew, associate publisher and executive editor