To the surprise of his Senate colleagues, Abraham A. Ribicoff (D-Conn.) annouced yesterday that he will not seek reelection next year.

Said Ribicoff, who is 69 and completing his third six-year Senate term: "The greatest thing for a man in public life is to learn how to step away at the height of his power."

In an interview in his office later, he added: "I think you've got to leave with grace and class."

Senate colleagues, Connecticut politicians and even Robicoff's staff had assumed would be a candidate for reelection. He is in excellent health, looks younger than his age, and long ago established a pace of work that enabled him to stay in good condition.

But Ribicoff revealed yesterday that his decision not to run again came in November 1974, on the night of his last reelection. "No one should feel indispensible," he said yesterday, adding that politicians his age should allow younger people a chance at higher office.

Ribicoff's retirement will set off a scramble among politicians of both parties in Connecticut. A popular governor in the 1950s, Ribicoff enjoyed enormous support, and it was unlikely that a serious opponent would have materialized in either party next year had he chosen to run again.

Two Democratic congressmen, Christopher J. Dodd and Toby Moffett, are likely contenders for Ribicoff's seat. Both are young, activist liberals who have been rivals for years. Another potential candidate is the state's Democratic governor, Ella Grasso.

On the Republican side, Rep. Stewart B. McKinney is a possible candidate, as is last year's unsuccessful GOP candidate for lieutenant governor Lewis Rome.

Ribicoff's retirement gives Republicans a good shot at a Senate seat they had written off as unwinnable.

At yesterday's news conference in the Russell Senate Office Building, Ribicoff said he had no specific plans for after January 1981 and that he expected the next year and a half to be the busiest time he has ever had in the Senate.

As chairman of the Governmental Affairs Committee and the subcommittee on international finance, Ribicoff will be at the center of a number of legislative battles. He will be the floor manager of the controversial and complicated new trade bill that should reach the Senate this year.

Ribicoff did say he was not interested in an ambassadorship: "I don't want to live abroad-ever." He recently bought a country estate in Cornwall Bridge, Conn., in the western part of the state.

Ribicoff has spent four decades in public service as a state legislator, judge governor, secretary of the Department of Health, Education and Welfare and a senator since 1963.

During that time he also has accumulated considerable wealth, largely because of successful real estate dealings, according to friends.

In recent years Ribicoff has told friends and colleagues privately that the Senate was changing in a way he disliked. There are few real statesmen left, he has often complained to intimates.

Ribicoff expressed cool support for President Carter at his news conference, saying he would support the president in 1980, "if he is the nominee of the Democratic Party," but adding, "I don't know who is opponents might be." Questioned later, he said he thought Carter "should be renominated."

Ribicoff is the fourth senator whose terms expire in 1981 to announce retirement. Others are Henry L. Bellmon (R-Okla.), Richard Schweiker (R-Pa.), and Adlai Stevenson (D-Ill).). CAPTION: Picture 1, Abraham and Lois Ribicoff after he announced he won't seek reelection next year. By James K. W. Atherton-The Washington Post; Picture 2, Ribicoff to Retire