Peter A. Derow, 39, has been named chairman of the board of Newsweek, Inc., the parent Washington Post Co. announced yesterday. Derow, who has been president of Newsweek since March 1978, will retain that title.

Derow succeeds Robert D. Campbell, 62, who is retiring as chairman at the end of this year after 30 years with the company.

In announcing Derow's appointment, Washington Post Co. President Mark J. Meagher said "Peter has played a major role in Newsweek's great strides over more than a decade, culminating in a 1979 performance that has broken all the magazine's records -- for advertising pages, revenues and operating income."

Derow joined Newsweek in 1965, serving in both general management and with Newsweek International. He first became president of Newsweek, Inc., in October 1976. After leaving nearly a year later to become senior vice president of CBS, Inc., he returned to the presidency of Newsweek in March, 1978.

Campbell, who has been chairman of Newsweek since October, 1976, will continue to serve the company as a consultant for the next five years. At the end of this year he will also retire from the positions of director and vice president of the Washington Post Company.