Newsweek magazine President Peter Derow resigned yesterday to become head of entertainment conglomerate CBS Inc.'s publishing group.

He was succeeded at Newsweek, a subsidiary of the Washington Post Co., by Mark Edmiston, who has been executive vice president of the magazine and head of business operations there since mid-1979.

Derow, 40, had been chairman and president of Newsweek as well as a vice president and director of the Post Co. He had been with Newsweek since 1965 except for a six-month period in late 1977 and early 1978, when he made an earlier move to CBS as a senior vice president in charge of corporate staff developments.

Post Co. Chairman Katharine Graham, who announced the choice of Edmiston yesterday, said of Derow: "We respect [his] decision to undertake responsibilities at a larger company, and we will miss him. He has played an important part in the growth of Newsweek . . ."

CBS publishing operations include the Holt, Rinehart and Winston book firm; Family Weekly, which is distributed with more than 350 newspapers; and a diverse collection of magazines that include Women's Day, Audio, Road & Track, Field & Stream and American Photographer.

Edmiston, 38, joined Newsweek in 1973 after working at Time Inc. and the Saturday Review. He was president of Newsweek's international division before taking over as head of domestic business operations in July 1979.

"Mark Edmiston is a talented and innovative executive who has established an outstanding record of achievement at Newsweek . . . With [Edmiston] as president and with Newsweek's worldwide journalistic resources, the company is well-positioned for vigorous growth in the 1980s," Graham said yesterday.

Newsweek publishes U.S. and international editions of the weekly news magazine, as well as the year-old Inside Sports monthly. The Post Co. subsidiary also operates video and book divisions. All Newsweek operations have been profitable except for Inside Sports, which lost some $12 million in its start-up period last year. Graham said yesterday the company's management is "encouraged by the solid progress" being made by the sports periodical.

In another Post Co. executive change, Alan Perris was named to a new position of executive vice president for programming and production of the firm's broadcasting subsidiary, Post-Newsweek Stations Inc., effective Aug. 1. Perris has been vice president and general manager of WPLG-TV in Miami, one of four television stations owned by the Post Co.

Perris, a Post Co. broadcasting executive since 1973, will devote a major part of his time for the balance of 1981 supervising a full study of video opportunities for the Post Co. as a whole, according to Post-Newsweek Stations President Joel Chaseman.