U.S. District Judge John J. Sirica, perhaps the nation's most famous trial judge because of his role in helping unravel the Watergate scandal, will step down from full-time active duty on the federal court here at the end of this month.

Sirica announced his decision to take "senior" judge status at the age of 73 and after 20 years on the bench in a letter to President Carter on Tuesday, the same day he took his final act in a criminal Watergate case and reduced the sentences of the thre major defendants.

In announcing Sirica's retirement yesterday, the White House released a letter to the judge from Carter praising Sirica for becoming "a lasting symbol of unflinching devotion to duty."

In the letter, Carter told Sirica that during the judge's "distinguished career in the federal judiciary since 1957, you have been given an opportunity afforded to few of us who enter upon public service: to "exhibit, at a time of greatest challenge to our system of government, the personal courage and wisdom needed to sustain it . . ."

Sirica is the third judge this year to step down from full-time active service on the 15-member federal trial bench here. On Tuesday, U.S. District Judge Howard F. Corcoran announced he would take senior status at the end of November and earlier this year U.S. District Judge William B. Jones moved into senior status.

Judges who take senior status can still sit on the bench as often as they like while continuing to draw their full salary and keep their staffs and offices. However, the decision creates a vacancy.

Although he has been eligible to take senior status since he turned 70. Sirica has remained a full-time active judge apparently in a desire to continue his involvement with the Watergate criminal cases until their end.

A man whose shy friendliness in private belies his gruff courtroom manner and the harsh sentences that have earned him the nickname of "Maximum John." Sirica began his involvement with the Watergate scandal on Sept. 15, 1972, when, as chief judge, he assigned himself to hear the criminal case against the original Watergate conspirators.

In his first 15 years on the bench, Sirica was not noted for outstanding legal scholarship but had earned a reputation as an oft-reversed, bluntly outspoken and independent judge. As do all federal judges here, he became involved in a wide array of criminal and civil cases of national and local importance.

He presided over an eight-month civil trial in the early 1960s that remains probably the longest trial ever conducted here, sentenced the imperial wizard of the Ku Klux Klan to jail for contempt of Congress in the late 1960s, presided over a trial in which a Georgetown interior decorator was convicted of being a fence for stolen property, presided over a trial of a man convicted of hiring someone to rape and kill the man's wife, and blocked the Three Sisters Bridge from being built.

But it was in the Watergate scandal that Sirica earned his national reputation. He did not believe that the whole story of Watergate came out at the original trial and conducted what he called a "search for truth" in his courtroom.

One of the defendants, James McCord, then wrote a letter to Sirica on the date of his sentencing that began to outline the scope of the cover-up that had been perpetrated by the White House.

Sirica enforced subpoenas for White House tapes that subsequently led to the downfall of President Nixon, and presided over the Watergate cover-up trial as well.

A former boxer who prides himself on his physical fitness. Sirica was stricken by a massive and near fatal heart attack in February, 1976, while speaking to an alumni group downtown.

He was hospitalized for several weeks and returned to the bench in the fall of 1976. He now appears healthy and has tried attempted to keep a full workload, but he confides to associates that he tires easily when sitting in court for long periods of time.

A native of Connecticut, Sirica - the sone of an immigrant barber - lived in Jacksonville, Fla., and New Orleans before coming to Washington in 1918.

He received his law degree from Georgetown University and has been a prosecutor and defense attorney here, as well as a congressional committee counsel. A Republican, he was chief of the trial department of the prestigious firm of Hogan and Hartson when he was nominated to the bench by President Dwight D. Eisenhower.

In his letter informing President Carter of his decision to become a senior judge, Sirica said that he hoped that in his 20 years on the bench "I have contributed to and measured up in some slight degree to the high responsibilities thus imposed."