Rifaat Assad, the strong-willed younger brother of Syrian President Hafez Assad who appeared to have been sent abroad to live in political exile, unexpectedly returned to Damascus today to a noisy welcome from his supporters.

His return amounts to a political bombshell in Syrian politics and is certain to be interpreted there as a reaffirmation of the Syrian leader's support for his brother as his successor in the race for the presidency. That race began a year ago, when the president fell ill with a heart ailment and there were fears for his life.

The subsequent jockeying for position led to a near full-scale military confrontation between Rifaat Assad's 25,000-strong Defense Companies and a coalition of Army and security forces opposed to him that turned Damascus into a checkerboard of rival armed groups.

After a number of minor clashes in Damascus and other cities, the Syrian president prevailed upon his brother to leave the country to ease the growing tensions.

Rifaat Assad left in early June for Paris with 40 bodyguards and his family and then took up residence in Geneva, an event interpreted at the time as marking the apparent end to his political career.

The Syrian president took advantage of his absence to put some units of the heavily armed Defense Companies under Army command and restore calm. It is not clear today whether this special security force, which crushed the uprising of Moslem fundamentalists in the northern city of Hama in February 1982 with enormous loss of life, still operates as a separate force.

In September, Syrian Defense Minister Mustafa Tlas said in an interview with the West German magazine Der Spiegel that Rifaat Assad was persona non grata in Damascus and would never be allowed to return.

Rifaat Assad apparently timed his return to coincide with the state visit of President Francois Mitterrand, the first French leader to visit Syria since its independence from France in 1946 and thus a major event being thoroughly covered by the French media.

As word filtered out in Damascus today that Rifaat Assad had returned, his supporters began firing automatic weapons into the air to indicate their happiness at his homecoming.

The events leading up to Rifaat Assad's return are still cloaked in mystery, but it is clear that he and his brother must have had secret contacts to arrange for his political rehabilitation. Two weeks ago, the official Syrian gazette published a presidential decree stating that Rifaat Assad was officially in charge of security matters, a job he should already have had by virtue of his position as vice president for security and military affairs.

This decree, in retrospect, indicated that agreement had been reached. When Rifaat Assad left Syria last June, there were reports circulating in the capital that he had already struck a bargain with his brother to leave Syria for a "cooling-off period" in return for which the president would support his bid for the succession. But most subsequent reports seemed to indicate that he had lost favor with his brother.

One report circulating here tonight said that the Saudi Arabian Crown Prince Abdullah had played a role in arranging Rifaat Assad's return and even persuaded the Syrian president to allow this before the opening of the ruling Baath Party congress in mid-December.

The crown prince and Rifaat Assad are close friends and are related through two of their wives who come from the same Shammar tribe.

The Baath Party congress is expected to elect a new political bureau, and Rifaat Assad would need to be prominent among its new members to consolidate his bid for political power.

Most of his rivals, many of whom, like him, are of the minority Alawite sect, have maintained their positions in the Army and the intelligence and security forces.