The body of Jean Seberg, 40, who left her native Iowa as a teen-ager to become a film star, and found fame, ridicule and tragedy, was discovered in Paris yesterday in the back of her car.

Paris police said Miss Seberg, who had been treated in psychiatric institutions on a number of occasions in recent years, had disappeared Aug. 30 from her apartment on the Avenue de Longchamp.

The body was covered by a blanket and it appeared that the actress had been dead for several days. An autopsy is to be conducted.

Miss Seberg, the daughter of a Marshalltown, Iowa, druggist, found herself at the age of 17 in the glare of international publicity when she was chosen by Otto Preminger over 18,000 others for the lead role in Saint Joan.

At the time, she was a student at the University of Iowa. "I packed my bags and left after being there just six days," she said.

Her theatrical training could be described only as rudimentary. She had appeared in five summer stock productions. At age 12 she took one of the works of Stanislavsky from her hometown library, but returned it unread.

If the real world imitated that so often depicted by Hollywood, the completion of Saint Joan might have turned the slender, 5-foot 3-inch blond into a star overnight.

But in fact the film failed and much of the blame was pinned on Miss Seberg, who was called "callow and unconvincing" and an "Iowa Amateur." Her notices in Bonjour Tristesse, her second film, were a little better, but not much.

A critic for a major New York newspaper suggested sending her back to "that Iowa high school whence she came." However, although criticized as incapable of registering emotion, Miss Seberg was not about to give up.

"The hardest time to quit," she said, "is when everybody says you should. I can't afford to fall back. I must fall forward. I'd have to learn a different craft and I don't know anything else to do." She took acting and miming lessons.

In August 1958, Preminger assigned her contract to Columbia Pictures and she played soon after in The Mouse that Roared (1959).

In September, 1958, she married Francois Moreuil, a Paris lawyer with connections in the French film industry. He introduced her to Jean-Luc Godard, who cast her in A Bout de Souffle (Breathless), which put to good use the talents she was then acknowledged to have.

The film was a hit, and Miss Seberg's career began to prosper, as she won increasingly favorable reviews in both French and American films, among them in the French Style, Lilith, and Moment by Moment.

Her personal life was stormy. After divorcing Moreuil in 1960, she married author and diplomat Romain Gary. Her third husband was American director Dennis Berry.

In 1970, shortly after separating from Gary, she had a stillbirth. Friends said she never recovered from the experience.

She wrote a book called "Blue Jeans" to describe the psychiatric treatment she underwent in recent years.

Of her unsteady fortunes she once said: "In my profession one must choose between a great career and a full life. I have chosen the adventure of life."