Joseph J. Sisco, former under secretary of state and a top expert in U.S. Middle East policy, has resigned as president of American University, and asked that he be given a new part-time post as university chancellor instead.

Sisco, 59, has served as head of American University since July 1976 while continuing to lecture and write on foreign policy. He reportedly wants to spend more time speaking and writing on foreign affairs while codntinuing to play a major role in university fund-raising.

Methodist Bishop James K. Mathews, chairman of American's board of trustees, confirmed Sisco's resignation in an interview last night. He said the trustees would meet Monday to decide whether to give Sisco the new role he has asked for, and also possibly to pick a new president.

Sisco also confirmed that he had resigned, but refused to discuss it.

Since Sisco became president, American University has adopted a policy of tougher standards for both admission and graduation in an effort to stem enrollment losses by raising the university's reputation for academic quality.

Sisco also has led a major fund-raising effort and completed a new university library, which had been planned for many years.

Last year he encountered some criticism from a group of trustees and faculty members for the time he spent away from the university as a lecturer, occasional television commentator, and writer of newspaper columns on foreign policy.

However, the criticism subsided when Sisco canceled his contract with a New York speaker's bureau through which he had earned about $80,000 during his first two years as university president on top of his university salary of about $50,000 a year.

Several trustees reportedly have urged Sisco to reconsider his resignation since he submitted it to Mathews last month. However, Sisco has indicated he will step down as president in early 1980 regardless of whether the trustees create the new post of chancellor, as he has requested, sources said.

"I think Sisco's heart now really is in the Middle East and foreign policy," one university official said yesterday, "and he just doesn't want to spend so much time any more on the university."

Another source said Sisco wanted to speak widely during the political campaigns of 1980, although he has denied having political ambitions.

Sisco entered the State Department as a career foreign service officer in 1951, and rose through the ranks to become the department's No. 3 official from 1974 to 1976. He played a major role in former Secretary of State Nenry Kissinger's shuttle-diplomacy in the Middle East.

Kissinger was on hand when Sisco was inaugurated as American's 10th president in an elaborate ceremony in October 1976. In his inaugural speech Sisco called for an era of academic rigor and complained that "the educational process [was] often ignored" in many university reforms of the late 1960s.

He said the only way American University, a private college with a small endowment and relatively high tuition costs could compete with low-tuition public colleges would be to offer "a top-quality education that's worth $3,000 a year.

Although American's enrollment fell by about 1,000 over three years to 12,500 full-and part-time students last spring, average college board scores of entering freshmen rose substantially. Academic standards were stiffened by addidng "literacy tests" in reading, writing and mathermatics, and requirements that students take five courses each semester instead of four, and pass a range of courses outside their major fields.

Provost Richard Berendzen, a 40-year-old astrophysicist who developed most of the new academic policies, has been prominently mentioned as Sisco's successor. CAPTION: Picture, JOSEPH J. SISCO . . . asks part-time post