Jean Marie Pierce, 57, the assistant controller of The Washington Post in charge of the credit department and a person whose competence and good humor were valued not only in this newspaper but by colleagues elsewhere in her field, was fatally injured New Year's morning in a traffic accident in Northwest Washington.

D.C. police said Mrs. Pierce was a passenger in an automobile driven by her husband, John R. Pierce, when the vehicle was struck on the passenger side by another car at the corner of Woodley Road and Wisconsin Avenue. Mrs. Pierce was pronounced dead at D.C. General Hospital. Her step-daughter, Barbara H. Pierce was admitted to Georgetown University Hospital and was listed in serious condition. Mr. Pierce was treated for mino injuries at Georgetown and released.

Police identified the driver of the second car as a 16-year-old youth. They said the Pierce automobile was hit as Mr. Pierce was making a left turn ont Woodley from Wisconsin. Police said the youth had been charged with driving under the influence of alcohol, speeding and running a red light.

Mrs. Pierce was born in New York City, grew up there and in Washington, and was graduated from old Western High School. During World War II, she worked at the old War Department.

She joined The Post in 1946 as an employe in the accounting department. About a year later she was assigned to credit work, and this became her career. It is a field that requires understanding, firmness and a fine sense of judgment, for a credit manager must know when and how to say "no" as well as "yes."

Mrs. Pierce, who took a number of business courses at the Department of Agriculture Graduate School over the years, was named assistant controller in charge of the credit department in the early 1970s.

"Well, you win some and you lose some," she used to say, and that was characteristic of the wry and straightforward way in which she went about her business.

Donald E. Graham, the publisher of the newspaper, said yesterday:

"Jean was one of the finest and most universally liked people at The Post. She was admired for her professional ability, but also had thousands of friends among the people whose paths crossed hers at the paper. She brought tact and grace and consideration to a very tough and demanding job and she was one of The Post's treasures."

The qualities that accounted for Mrs. Pierce's success at The Post were recognized among professional organizations in which she was active. In 1979, she was named national president of the Advertising Media Credit Executives Association and hosted its convention here last year. She also was an officer of the Credit Management Associations of Greater Washington and was due to become its president this year. The organization is part of a national group that has 43,000 members.

Mrs. Pierce and her husband met while working at The Post. "Jack" Pierce joined the newspaper in 1948 and retired in 1977 as administrative assistant to the controller. They were married in 1959.

In private life, Mrs. Pierce was a lover of opera, ballet and the theater.

She was an omnivorous reader. Her choice for light reading was spy stories.

In addition to her husband and stepdaughter, both of Washington, survivors include a brother, Edward Griffin, also of Washington.

The family suggests that expressions of sympathy be in the form of contributions to the Washington Animal Rescue League, 71 Oglethorpe St. NW., Washington, D.C.