Klaus Fuchs, 76, the German-born nuclear physicist whose 1950 arrest for giving atomic secrets to Soviet agents became one of the most sensational espionage cases of the Cold War, died yesterday in East Germany, the official state news agency announced. It did not give the cause or place of Dr. Fuchs' death.

Dr. Fuchs was closely involved with the development of the atomic bomb in Britain and the United States during the 1940s. He confessed to giving details of the project to Soviet agents during a seven-year period.

He served nine years of a 14-year jail sentence in England, then flew to East Berlin on his release in 1959. There he became deputy director of East Germany's nuclear research institute at Rossendorf, near Dresden, and a member of the central committee of the East German Communist Party.

Dr. Fuchs always insisted that he had spied only because of his belief in the rightness of communism. He said the only money he ever accepted was a one-time payment of 100 British pounds as a way of binding himself to the cause.

His case led to the exposure of an espionage ring that included atomic spies Julius and Ethel Rosenberg, who were executed in New York in 1953, and Harry Gold, a Philadelphia biochemist who was Dr. Fuchs' contact with the Soviets when Dr. Fuchs was working in the United States from 1943 to 1945. Gold drew a 30-year prison sentence.

Their information advanced the Soviet atomic weapons program at least 18 months, according to the Senate-House Atomic Energy Committee, and it helped the Soviets explode their first atomic bomb in 1949.

It also seriously damaged the exchange of nuclear information between the United States and Britain. This situation was made worse by the defection in 1951 of Guy Burgess and Donald McLean, high officials of the British diplomatic and intelligence services. All this contributed to an atmosphere bordering on anticommunist hysteria in the United States in which Dr. Fuchs, the Rosenbergs, Gold, Burgess and McLean were seen as incarnations of the devil.

In 1952, the Saturday Evening Post published a four-part series on Dr. Fuchs, calling him "the world's worst traitor in nearly 2,000 years." The Atomic Energy Committee said, "Fuchs alone has influenced the safety of more people and accomplished greater damage than any other spy not only in the history of the United States but in the history of nations."

Slender, bookish and intense, Dr. Fuchs appeared to be the model of a dedicated scholar. He had been a Communist since the early 1930s when he joined the party to oppose the rising Nazi movement in Germany.

He was born in the German village of Russelsheim, near Frankfurt, where his father, Emil Fuchs, was a professor of theology and a member of the Religious Society of Friends. The elder Fuchs opposed the Nazi regime and was eventually sent to a concentration camp.

Dr. Fuchs studied at the universities of Leipzig and Kiel. He fled to France after the Nazis came to power in Germany in 1933 and moved to Britain a few months later.

He studied at Bristol University, where in 1937 he received a doctorate in mathematical physics. He won a research scholarship at the University of Edinburgh, where he received a second doctorate in theoretical physics.

As a German alien, he was interned by the British and sent to Canada on the outbreak of World War II. In 1941, he was returned to Britain and assigned to work on the British atomic bomb project at the University of Birmingham. He became a naturalized British subject in 1942.

Within a few months of his return to England, Dr. Fuchs sought out a Soviet agent through his Communist Party connections and began to pass along secrets to the Soviets. "At this time I had complete confidence in the Russian policy and I believed the Western Allies deliberately allowed Russia and Germany to fight each other to death. I therefore had no hesitation in giving all the information I had," Dr. Fuchs said in his confession to British authorities.

In 1943, Dr. Fuchs came to this country to work on the U.S. atomic bomb and was assigned to research facilities in New York and Los Alamos, N.M. He was able to pass along to the Soviets details of the plutonium and the uranium bombs and to describe the design and method of their construction.

After the war, Dr. Fuchs returned to England and became deputy scientific director of the British Atomic Energy Research Institute at Harwell. He held that job until 1950 when an FBI investigation in the United States uncovered evidence that a British scientist had passed atomic secrets along to the Soviets. British investigators arrested him and he confessed.

Described as a model prisoner, Dr. Fuchs had five years taken off his sentence for good behavior, and he was put aboard a plane for East Germany immediately on his release.

In 1979, he retired from East Germany's nuclear research institute.

In a 1960 interview with The New York Times, Dr. Fuchs insisted that given a chance, he would do the same things again.

"The Soviet Union is on the right line. It is for peace. Whatever helps the Soviet Union is right," he said.

In 1959, Dr. Fuchs married Greta Keilson, a longtime Communist. The East German news service did not list any survivors.