Alexander I. Solzhenitsyn has retained Edward Bennett Williams, the Washington trial lawyer, to help defend a recently imprisoned dissident in the Soviet Union, Williams and Solzhenitsyn revealed yesterday.

The man Williams will try to assist is Alexander I. Ginzburg, 40, whom the KGB arrested on Feb. 4. Ginzburg is a close friend of Solzhenitsyn and his wife, Natalia.

Shortly before his arrest Ginzburg announced publicly that he had distributed more than $360,000 contributed by Solzhenitsyn to the families of political prisoners in the U.S.S.R.

Williams said yesterday he did not know what he would be able to do on Ginzburg's behalf. "It's a brand new idea," he said, describing the prospect of an American lawyer representing a Soviet dissident in a Soviet criminal case.

"It's perfectly clear that they [the Soviet authorities] are not going to let me participate in a trial," he said. But he said the human rights clauses of the European security agreement signed in Helsinki in 1975 might provide some basis for legal action. Williams said he would apply for a visa to go to the Soviet Union and see his new client "at the appropriate time."

The Carter administration's willingness to speak out on human rights issues apparently influenced Solzhenitsyn's decision to hire Williams to defend Ginzburg. Williams said Solzhenitsyn "believes the President" when Carter speaks out for human rights "across the world and behind the Iron Curtain."

As part of the new posture on human rights questions, the Carter administration has already spoken out on Ginzburg's behalf. Three days after his arrest the State Department expressed "profound concern" for him.A department spokesman said Ginzburg "seems to have been singled out for especially harsh treatment."

Williams spent the day Saturday with Solzhenitsyn in Cavendish, Vt., where the Nobel Prize-winning author now lives. Solzhenitsyn told him he feared Ginzburg could not survive if he is imprisoned again.

Ginzburg has twice served prison sentences in the past, and suffers from ulcers. He had pneumonia at the time of his arrest in a phone booth outside his apartment near Moscow, according to friends there.

Reports from Moscow over the weekend quoted dissident sources as saying Ginzburg is being charged under Article 70 of the penal code which covers "anti-Soviet" behavior. He was previously convicted under this article, so he could now be liable to a sentence of 10 years.

In 1961 Williams represented a Soviet employee of the United Nations who was arrested in New York on charges of espionage. In that case Williams was retained by the Soviet government. His client was eventually allowed to leave the country without being tried.

Ginzburg has een held incommunicado at a prison in Kaluga outside Moscow since his arrest. The KGB often takes many months to prepare a case against such political prisoners before bringing them to trial.

Williams said yesterday he had not discussed the question of a fee for his work with Solzhenitsyn, and he declined to say whether he would ask to be paid.