William D. Broyles Jr. will replace Lester Bernstein as editor of Newsweek, the magazine announced yesterday.

Broyles, 37, currently editor-in-chief of Texas Monthly and California magazines, will take charge on Sept. 7, becoming the fourth editor of the newsweekly in the past 10 years.

In a statement, Katharine Graham, chairman of The Washington Post Co., which owns Newsweek, praised Broyles' "understanding of the critical issues we face today, his innovation in editing and graphics and his outstanding editorial record" at the regional magazines.

She also congratulated Bernstein, 61, for building "a solid foundation for future growth," and cited the record number of awards received during his tenure, including two National Magazine Awards this past April, one for general excellence. Bernstein, named editor in June 1979, could not be reached for comment yesterday, but said in the statement that "I believe I've fulfilled the traditional role envisaged" and that after an "orderly transfer" he will resign "to pursue plans of my own."

Rumors of the impending turnover had circulated for weeks, as Bernstein suggested in his inter-office memo yesterday--"not exactly first with the news, but at least it's official." But some staff members were surprised at the choice of an editor with relatively limited news experience. Broyles worked briefly as a reporter for the Houston Post and correspondent for the London Observer before cofounding Texas Monthly 10 years ago.

Broyles attended Rice University and Oxford, where he took a master's degree in philosophy, politics and economics. After serving in the Marine Corps in Vietnam, he taught philosophy at the U.S. Naval Academy and was assistant superintendent of the Houston public schools before founding Texas Monthly with Michael Levy in 1972.