DUBAI, United Arab Emirates (AP) _ The United States defeated the world champions, the Soviet Union, 2.5-1.5 in the eighth round of the 27th chess olympiad after the final game in their match ended in a draw today.

Andre Sokolov, the third-ranked player in the world, was unable to break through the defense of Larry Christiansen, a 30-year-old grandmaster from Pasadena, Calif., the United States' No. 2 player.

After a marathon, 85-move struggle, Sokolov finally offered a draw which gave the U.S. team victory.

The Soviet defeat followed the dramatic victory Sunday night of American champion Yasser Seirawan over the world champion, Garri Kasparov.

Kasparov pressed too hard for a win, and in the resulting complications Seirawan gained an unstoppable phalanx of pawns on the king-side.

Kasparov resigned one hour after the game was adjourned Sunday evening.

''Of course, I'm very pleased,'' commented a delighted but restrained Seirawan after his victory, accompanied by U.S. team captain John Donaldson and team member Lubosh Kavalek. ''It's unexpected and I was lucky. I'm already thinking about my next game.''

On board three, Kavalek, a 43-year-old Czechoslovak emigre from Washington, D.C., drew effortlessly with Soviet grandmaster Artur Yusupov.

On board four, 20-year-old Maxim Dlugy of Leonia, N.J., played to a draw with the disadvantageous black pieces against Rafael Vaganian.

Dlugy, a former world junior champion, emigrated with his family from the Soviet Union when he was 10.

Both Kavalek and Dlugy conducted their postgame analyses with their Soviet opponents in Russian.

The U.S. team is now placed second, equal with Hungary at 22.5 points, ahead of the Soviets, but a point behind the leaders, England.

At the last olympiad, held in Salonika, Greece, in 1984, the U.S. team also defeated the Soviets, 2.5-1.5, but were playing a weakened Soviet team.

Both Kasparov and former world champion Anatoly Karpov were in Moscow then, engaged in a bitter personal and professional tussle for the world championship crown.

In Salonica, the U.S. team finished fourth, but at this event it is better placed, with Hungary scheduled as their ninth round opponent today.

Three American grandmasters refused to be considered for the U.S. team to protest Israel's exclusion from the event, which is biled in Dubai as the biggest sporting spectacular ever held in an Arab country.

The three who refused to come were all from New York - 23-year-old Joel Benjamin; grandmaster Dmitri Gurevich, a Soviet emigree; and Soviet defector Lev Alburt, a former U.S. champion.

Seirawan, 26, from Seattle, Wash., was born in Syria, but grew up in the United States. He is standing for the post of president of the U.S. Chess Federation. In 1982, Seirawan defeated Karpov, who was then world champion, at a tournament in London.