Walter Hallstein, 80, the first president of the European Common Market and a tireless advocate of European unity, died Monday in Stuttgart, Germany. The cause of death was not reported.

Mr. Hallstein was a lawyer by training and a diplomat and international civil servant by profession. In the 25 years following the end of World War II, he made a reputation as one of the most single-minded as well as the most influential of Europe's statesmen. A cold and precise man in negotiation, he was passionate in his pursuit of a unified political system embracing his native Germany and her wartime friends and foes in the West.

In the early 1950s, Mr. Hallstein was minister of state in the government of Chancellor Konrad Adenauer. As such, he was the ex-officio German foreign minister. He helped negotiate the European Coal and Steel Community, a forerunner of the Common Market, and became a close friend of Jean Monnet, "the father" of the Common Market. He played important roles in establishing diplomatic relations with Israel and the Soviet Union. In the latter case, Moscow agreed to release thousands of German prisoners of war.

In the same period, he developed the "Hallstein doctrine" to reunite Germany. The doctrine stated that West Germany would sever ties with any government that recognized East Germany. When it went into effect in 1955, Bonn severed relations with Yugoslavia and a number of other countries.

The exception to this was the Soviet Union and its allies in Eastern Europe. Mr. Hallstein and Adenauer argued that Germany must maintain relations with Moscow since the Soviets were the only ones who could prevent East Germany from joining West Germany.

Following the ratification of the Treaty of Rome, which set up the Common Market, Mr. Hallstein was elected president of its commission. He took office in 1958 and held it until June 30, 1967.

He directed his energies toward promoting ever-closer relations between the member countries and installing supranational institutions to harmonize their policies. He favored the entry of Britain, a project which was vetoed by President de Gaulle of France. He favored a European national defense force and a European parliament. In this way, he believed, Western Europe could play an independent role in world affairs.

His plans were shattered by de Gaulle. The issue was an effort by Mr. Hallstein to create a European federation. De Gaulle ordered the French representative to the Common Market to boycott meetings. This had the effect of blocking the plan.

It also established a precedent allowing any member of the community to veto actions approved by others.

Mr. Hallstein returned to Germany and was elected to the West German parliament as a Christian Democrat. In 1969, he opposed ratification of the treaties normalizing Bonn's relations with the Soviet Union, Poland and East Germany. The treaties, negotiated under Chancellor Willi Brandt, a Social Democrat, as the linchpin of his policy of "ostpolitik," effectively ended the "Hallstein doctrine."

At the news of Mr. Hallstein's death, flags were ordered flown at half-staff at the headquarters of the Common Market in Brussels. Gaston Thorn, the president of the community, spoke of Mr. Hallstein as a man of courage and a singular breadth of spirit.

In Bonn, Karl Carstens, the president of West Germany, said his country had lost a man dedicated to the ideal of European unity.

Mr. Hallstein was born at Mainz, Germany. He studied law at the universities of Bonn, Munich and Berlin and later became a professor of law. During World War II, he was drafted into the German army. Captured by the Americans, he began to teach economics and other subjects to his fellow prisoners. He caught the eye of Adenauer after the war and his career was launched.

A spare, bespectacled man, Mr. Hallstein often was referred to as "the German professor." He never married and lived alone in Stuttgart after his retirement from politics in 1972.