S. Dillon Ripley, who has presided over two decades of dramatic change and expansion at the Smithsonian Institution, will retire next year.

The news that Ripley would be leaving his job as the Smithsonian's secretary came in an announcement yesterday that he would be staying--for a year beyond his 70th birthday in September.

"After nearly two decades of dedicated, active and highly successful service, he has now generously agreed to extend his service even further to bring about an orderly change of leadership," said Chief Justice Warren E. Burger, who is chancellor of the Smithsonian, in an official statement.

A search committee headed by Princeton University President William G. Bowen will look for a successor.

"It's a perfectly normal transition," said Carlisle H. Humelsine, chairman of the executive committee of the Smithsonian's board of regents. Longstanding Smithsonian retirement policy calls for "normal retirement" at 65 and "mandatory retirement" at 70, Humelsine said, "unless the regents ask for a delay."

"In 1977, when Dillon reached 65, the regents asked him to stay on because we had a number of things in the works," Humelsine said.

Last year the regents began discussing the succession issue, he added, and they came up with the September 1984 timing to get the South Quadrangle complex, Ripley's last major project, well along the road to completion while still moving early enough to assure the continued presence of Smithsonian Undersecretary Phillip S. (Sam) Hughes, now 66.

"Sam is a very, very important element in the Smithsonian situation," Humelsine said. "The regents felt very strongly that they wanted Sam Hughes to stay on and be a part of the transition period. We also felt that it wasn't going to be any easy job to replace Dillon Ripley, so we wanted to allow plenty of time."

Despite his modest-sounding title of secretary, Ripley earns $92,500 a year as chief executive of a domain that includes not only the museums along the Mall but the National Portrait Gallery, the National Museum of American Art, the National Zoo, the Cooper-Hewitt Museum in New York and such far-flung outposts of scholarship as the Smithsonian Tropical Research Institute in Panama and the Smithsonian Astrophysical Observatory in Cambridge, Mass.

Approximately 4,500 people work for the Smithsonian, and more than 20 million people visit it annually. It has an annual operating budget of about $250 million.

An ornithologist with degrees from Yale and Harvard, Ripley left his job as director of Yale's Peabody Museum and came to the Smithsonian in 1964. He started right in implementing his vision of a museum--a playful, unintimidating meeting place for people and knowledge. At his instigation, everyday subjects, life-like displays, bright colors, live performances, films and even a carousel became part of the scheme of things on the Mall. He was criticized for vulgarizing a sacred swath of grass, but the criticism--that criticism, anyway--tended to fade as attendance and enthusiasm swelled.

"I said, 'You know, these museums are living places,' " Ripley recalled in an interview last fall. "Children of all ages are here. They come to learn. They must learn because it's fun . . . They shouldn't be footsore and weary.

"I wanted to have Dutch music wagons and monkeys and hurdy-gurdies, and make the place a living experience, especially in the summer months. And that's what spawned my idea for the Folklife Festival--that we should take the objects out of the cases and make them sing . . . But there were people in the congressional hearings who appeared to be quite scandalized."

Ripley also threw his weight behind plans that gave rise to the Museum of American History, the Anacostia Neighborhood Museum, the Cooper-Hewitt Museum, the Museum of African Art and the hugely popular Air and Space Museum. And he played a central role in obtaining funds for the Hirshhorn Museum and Scupture Garden and, more recently, the $75 million South Quadrangle complex that will house underground museums of Asian and African art.

Ripley's patrician manner and globe-hopping ways have made him his share of enemies, and embarrassing stories have surfaced at regular intervals about such issues as the transfer of birds from the National Zoo to his private aviary in Litchfield, Conn., and the use of his influence to boost the ornithological career of his son-in-law. There have been weightier charges, too--most concerning Smithsonian finances and personnel procedures.

In 1977, the General Accounting Office issued a report accusing the Smithsonian of systematically trying to conceal its finances from congressional oversight. In March, a House subcommittee questioned the management of the Smithsonian's world-famous mineral and gem collection under former curator Paul Desautels. The Smithsonian announced policy changes in reply to both inquiries.

But Humelsine said these issues had not figured in yesterday's announcement. "There were no negatives whatsoever as far as the regents were concerned--quite the contrary," he said.

Ripley will become secretary emeritus upon his retirement, and, like other past secretaries of the Smithsonian, he will continue to do research there--in his large ornithological laboratory at the Museum of Natural History.

The search committee has not yet begun the process of list-making, according to Humelsine. It has not even decided if it will continue the tradition of having a working scientist as secretary, he said. "I think it's pretty obvious that the first thing the search committee will direct its attention to is trying to really define what we're looking for," he said.

One Smithsonian official, demanding anonymity, said yesterday that Ripley's departure "had absolutely been expected. Sam Hughes has really been managing this place for quite a while, while Dillon handles the South Quad. That will be Ripley's final monument, and all his energies have been directed to it, and it has been generally assumed that when he got as far along as he could, he would go--or get ready to go."

Last fall, Ripley said he was busier than ever, "running around from pillar to post primarily trying to raise money and see to all our differing affairs at the same time.

"I suppose that one of my major strengths is that I have good circulation," Ripley said then. "You know, blood pressure and all. That is a strength because it keeps me going and I don't get as tired as some people do . . .

"My weaknesses, I think, are natural character weaknesses everybody has in proportion. I try to keep an even tone and balance and not get angry. Sometimes I get frayed out, and sometimes I'm more rude than I should be to people, but in general I have a fairly good degree of cool."

Ripley was at a White House reception honoring artists and art patrons yesterday, and when asked to speculate about his successor, he pleaded ignorance. "I don't know," he said. "It takes a long time to do something like that. I'll be on duty until September 1984."

At the same reception, William Bennett, chairman of the National Endowment for the Humanities, was more forthcoming. He volunteered the name of Charles Blitzer, former Smithsonian assistant secretary for history and art, as a possible candidate. Blitzer now is director of the National Humanities Center in North Carolina.

Ripley, a fervent conservationist, was more interested in discussing the threat posed by the possible construction of an airstrip on an island near Pitcairn Island, where the crew of HMS Bounty settled 200 years ago.

"It's essential that the island be saved," Ripley said. "It's a great nesting place for sea birds."