President Reagan yesterday nominated veteran assistant U.S. Attorney John A. Terry to a seat on the nine-member D.C. Court of Appeals, the city's highest court.

Terry, who is 49 years old today, has been chief of the appellate division of the U.S. Attorney's office since 1969. If confirmed by the Senate, he would fill a seat vacated by his current boss, U.S. Attorney Stanley S. Harris, who left the appeals court in February.

The long-time prosecutor's nomination was praised yesterday by defense lawyers who have opposed him and by several current judges on the court, all of whom know Terry because he has appeared before them about 100 times in his job as chief advocate for the U.S. attorney.

The defense lawyers described Terry, a Yale graduate who attended Georgetown law school, as "fair and a real professional," predicting that no matter what legal philosophy Terry may hold, he would, as one defense lawyer put it, "give us a fair shake."

D.C. Court of Appeals Chief Judge Theodore R. Newman Jr. called Terry a "lawyer of outstanding ability who has done an outstanding job" in arguing cases before the appeals court. "I look forward with keen anticipation to his coming to the court," Newman said.

Terry yesterday said he is "pleased and honored" by the nomination to serve a 15-year term on the appeals bench. He declined to comment on his views on home rule -- an issue that has at times bitterly divided the court -- or on legal issues in general.

Court observers who know him say they are not sure what impact he would have on a court that has been split in the past on criminal law questions as well as in interpreting the authority of Washington's government under home rule.

Newman and associate judges Julia Cooper Mack, Catherine B. Kelly and John M. Ferren generally form the liberal wing of the court. Judges William C. Pryor, John W. Kern III and James Belson are viewed by observers as more in the center of the court. Judge Frank Q. Nebeker and Harris were considered more on the conservative wing.

It is not clear where in this alignment Terry will fall."Nobody can ever predict what a robe will do to somebody," one judge cautioned.

During law school, Terry worked as a research assistant on a Senate commission chaired by the late Senator John L. McClellan (D-Ark.), investigating organized crime in the labor movement.

He joined the U.S. Attorney's office in 1962 and remained there until 1967, when he went into private practice for two years. He returned to the prosecutors' office to replace Nebeker as the chief of the appellate division.

Terry, was born in Utica, N.Y., has been a member of the D.C. Bar Board of Governors since 1977, a position he will have to resign if his nomination is approved.

He is expected to have no trouble making the 10-foot move from the lawyers' podium at the court of appeals to the other side of the bench. "He'll just be on the other side of the bench doing the same thing he's been doing for years," one judge said.

Criminal law, in which Terry is regarded as an expert, will be the one area in which he will not be allowed to work for perhaps a year, some judges pointed out. He will be disqualified from hearing any cases he worked on while he was at the U.S. Attorney's office. That means Terry will spend a long time focusing on utility rate cases, zoning appeals, divorce cases and juvenile cases.