ROCHESTER, Minn.--Supreme Court Justice Harry A. Blackmum '29 best known for the landmark 1973 ruling legalizing abortion, is undergoing treatment for prostate cancer at the Mayo Clinic a clinic spokesman said yesterday.

The clinic called the cancer "a small and localized recurrence."

"The justice is undergoing treatment which is not incapacitating and is short term," said spokesman Michael O'Hara. "The prognosis is excellent."

O'Hara said the cancer was detected during a routine examination earlier this month. He said Blackmun was in Rochester but was not hospitalized.

Blackmun, 78, was a guest at a hotel in Rochester, a switchboard operator said. He did not immediately return messages left at the hotel.

The clinic and the Supreme Court public information office declined to provide further details of his case.

Blackmun had his prostate gland removed Nov. 14, 1977, because of cancer. Mayo spokeswoman Meg Black said the new cancer was in the same area.

Blackmun's Washington office had said Friday that Blackmun was recuperating from minor hernia surgery at his vacation home in Wisconsin. His office said he had been swimming and had taken walks after the hernia surgery and made no mention of cancer.

Prostate cancer is the third most common cancer in men, after skin cancer and lung cancer, according to the American Cancer Society. About one of 11 men will develop prostate cancer at some point in his life, the society says.

The society estimates that this year, prostate cancer will be diagnosed in 96,000 men and claim 27,000 lives.

Blackmun is the third-oldest member of the court after Justices William j. Brennan, 81, and Thurgood Marshall, 79. The others are Byron R. White, 70; Chief Justice William H. Rehnquist, 62; John Paul Stevens, 67; Sandra Day O'Connor, 57 and Antonin Scalia, 51.

Lewis F. Powell, who will be 80 on Sept. 19, announced his retirement last month and President Reagan has nominated Robert H. Bork, a conservative federal appeals court judge, to replace him. Powell had his prostate removed in 1985 because of cancer.

Appointed to the high court by President Richard M. Nixon in 1970, Blackmun often sided in his early years on the bench with Warren Burger, his boyhood friend who became the staunchly conservative chief justice.

Blackmun won unanimous Senate confirmation after two earlier Nixon nominees--G. Harrold Carswell of Florida and Clement F. Haynsworth Jr. of South Carolina--were rejected after bitter debate. He took the court seat of Justice Abe Fortas, who resigned during controversy over the ethics of some of his off-the-bench activities.

By the mid-1970s, Blackmun had become known as an independent voice, sometimes voting with the diminishing liberal contingent and sometimes with the growing conservative group.