Harold S. Geneen resigned today as chairman of International Telephone & Telegraph Corp., effective Dec. 31. The giant communications concern's board named ITT predsident Rand V. Araskog, 48, to succeed Geneen.

The resignation of Geneen, 69, capped several months of maneuvering in the executive suite of ITT, the nation's 11th largest company. ITT president and chief executive officer Lyman C. Hamilton Jr. was replaced last July, after 28 months in that post, by Araskog, who had been senior vice president and chief operation officer of the company.

Hamilton had been viewed as Geneen's hand-picked successor. But sources said Geneen had been displeased with Hamilton's tenure as president and shunted him aside in favor f Araskog.

Geneen said yesterday that Araskog had shown his ability to run the company in his four months as president.

"I have tremendous confidence in the way Rand Arakog is running the company and for its future under his direction," Geneen said in a statement. "I think it is time for him to assume the added responsibilities of the chairmanship."

Geneen became president, chief executive and an ITT director in 1959 and was named to additional post of chairman in 1965. He gave up the chief executive's position in December 1977 and was replaced by Hamilton, who was assumed to be the eventual heir to Geneen's chairman post.

After 18 months as chief executive, however, Hamilton was replaced by Araskog last summer in a coup widely believed to have been engineered by Geneen. Several high-ranking ITT officials since have left the company.

Geneen, who will be 70 in January, also will give up his posts as chairman of the executive and policy committees on ITT's board, Those spots will be filled by Araskog. The company said he would remain a member of the board and be elected chairman emeritus.

ITT said the action was taken at Geneen's request. The announcement coincided with the disclosure ITT suffered a loss in the third quarter, the company's first loss while headed by Geneen.

ITT had sales last year of $19.4 billion and earnings of $662 million.