Walter J. Stoessel Jr., 66, a career Foreign Service officer who served as U.S. ambassador to Poland, the Soviet Union and West Germany and as deputy secretary of state during 1982, died of leukemia Dec. 9 at Sibley Memorial Hospital.

Mr. Stoessel was the senior member of the Foreign Service when he retired late in 1982 after a 40-year career. His reputation was that of a calm and unflappable diplomat who was well versed in the intricacies of his calling and who preferred a low public profile.

He was ambassador to the Soviet Union from 1974 to 1976. He spent much of his career on Soviet affairs beginning in 1947, when he was posted to Moscow as second secretary at the U.S. Embassy. After two years in that assignment he returned to the United States and spent a year at the Russian Institute at Columbia University. He then spent four years as a Soviet desk officer at the State Department. From 1963 to 1965, he was deputy chief of mission in Moscow.

Fluent in Russian, Mr. Stoessel could communicate with Soviet leaders in informal meetings and at official functions. This was particularly vital during the early months of his assignment as ambassador, a period that coincided with the unsettling transition from the Nixon to the Ford presidency. The Kremlin looked to him for assurances that there would be no disruption in U.S. policy toward the Soviet Union.

As ambassador, Mr. Stoessel helped negotiate agreements on limiting the size of nuclear tests, and he sat in on meetings between Secretary of State Henry Kissinger and Soviet leader Leonid I. Brezhnev late in 1974, a time of broadening dialogue between Moscow and Washington.

His tour in Moscow also coincided with a period in which the Soviets bombarded the U.S. Embassy with high levels of microwave beams thought by American officials to be part of an intelligence operation. However, they were never fully explained.

Mr. Stoessel got to know Kissinger in 1959 when he spent a year studying under him at Harvard University. Ten years later, while serving as ambassador to Poland, he met four times with representatives of China, opening the way for Kissinger's secret negotiations that resulted in President Richard Nixon's trip to China in 1972.

In 1972, having completed four years as ambassador in Warsaw, Mr. Stoessel returned to Washington as assistant secretary of state for European affairs. This assignment involved negotiations with Common Market nations on declarations aimed at revitalizing U.S.-European relationships.

He was ambassador to West Germany from 1976 to 1980, then returned to Washington as under secretary of state for political affairs, the third-ranking position in the State Department. A year later, on the recommendation of Secretary of State Alexander M. Haig, Mr. Stoessel became deputy secretary, the department's No. 2 post.

During the next several months his activities ranged from diplomatic missions to iron out details of the removal of Israeli settlers from the northern Sinai Peninsula to public criticism of the Soviet Union for using chemical warfare in Afghanistan. He served briefly as acting secretary of state after Haig's resignation in June 1982. He retired soon after George P. Shultz took office later that year.

Mr. Stoessel later undertook special assignments, including the leadership of a U.S. delegation to an East-West forum on culture in Budapest and meetings in March with Polish leader Gen. Wojciech Jaruzelski and other senior Polish officials.

Born in Manhattan, Kan., Mr. Stoessel grew up in Massachusetts and California and graduated from Stanford University. He joined the State Department in 1942 and was assigned to Venezuela before serving two years in the Navy during World War II. He rejoined State in 1946.

Mr. Stoessel also had served as political adviser to Supreme Headquarters Allied Powers in Europe in Paris and as deputy assistant secretary of state for European affairs before his ambassadorial appointments.

Survivors include his wife, Mary Ann Ferrendou Stoessel of Washington; three daughters, Katherine Kagan of New York City, Suzanne Seitz of Mount Vernon, N.Y., and Christine Zeydler-Zabrowski of Warsaw; one brother, James H. Stoessel of Concord, Mass.; one sister, Barbara Embree of Santa Monica, Calif., and three grandchildren.