Ballet superstar Mikhail Baryshnikov, who left the American Ballet Theatre a year ago to join George Balanchine's New York City Ballet, will return to ABT as its director effective Sept. 1, 1980.

Baryshnikov, 31, will continue as a principal in the New York City Ballet company "until my new duties become too time-consuming," he said in a prepared statement. He will rejoin ABT as a principal, thus becoming the third actively performing artistic director of a major company (the others are Alicia Alonso of the Cuban National Ballet and Marcia Haydee of the Stuttgart).

Baryshnikov's appointment was announced at the regular meeting of ABT's board of governing trustees in New York yesterday. Baryshnikov and Rudolf Nureyev, among others, had been reported candidates for the position since longtime ABT co-director Lucia Chase announced last January that she would retire in 1980.

In his statement, Baryshnikov said, "I have accepted the challenge of this position in the hope that I might bring something worthwhile to it. What that will be exactly I am not prepared to say at this point, since it is much too early. Also, I do not believe in making statements which later come back to haunt one."

Baryshnikov said he plans on "continuing to expose the formidable array of star at its most effective, and trying to find new choreographers and new choreograph, in addition to preserving the great classics."

Baryshnikov also expressed his appreciation to Balanchine and Jerome Robbins and company director Lincoln Kirstein for allowing him to continue dancing with the New York City Ballet for the time being. There was no statement from the New York Company.

Baryshnikov, who defected from Leningrad's Kirov Ballet in the spring of 1974, joined ABT several months later as a principal. In the next four years, he danced in 26 roles and choreographed two ballets, "The Nutcrackers" in 1976 and "Don Quixote" in 1978 - both of which premiered in Washington.

When he left the star-oriented ABT for the New York City Ballet, where Balanchine's dislike for the very word "star" is legendary, Baryshnikov was rumored to be in line to succeed the 75-year-old Balanchine as director.

(Baryshnikov's departure from ABT ironically stranded his sometime partner Gelsey Kirkland, who had moved over from the New York City Ballet to join him.)

Despite his respect for Balanchine, Baryshnikov has reportedly been restless at the New York City Ballet for several months. (At the time of the announcement yestarday, Balanchine's office said he wax in the hospital recovering from surgery.)

"I am surprised" at the appointment, said ballet grande dame and former ABT choreographer Agnes DeMille yesterday. "It's a proudly American company, and you'd expect an American director, wouldn't you?

"He's very great dancer," DeMille continued, "But I can't say anything about his administrative abilities - I just haven't experienced them."

Kennedy Center executive director Martin Feinstein said that in his conversations with members of ABT, "They demonstrated a considerable affection and enormous respect for Misha.

"Baryshnikov is a bright, intelligent, knowing an perceptive artist, and he has many of the qualities that could make him a great director," added Feinstein.

In assuming the direction of ABT, "Misha," as he is called, will be following in the footsteps of Lucia Chase, who has been co-director and the dominant figure in the company since 1945. Chase, who was the model for the hard-driving ballet company head in the movie "The Turning Point," was a founding member of ABT (then the Ballet Theatre) in 1940, and in the first seven years alone was reported to have sunk $2 million of her own money into the fledgling company.

Under Chase's leadership, ABT employes some of the finest choreographers of the century: Agnes DeMille, Jerome Robbins, Antony Tudor, Alvin Alley, Twyla Tharp. CAPTION: Picture, Mikhail Baryshnikov