Helene A. von Damm, a one-time Austrian farm girl who serves as U.S. ambassador to her native country, announced her intention to step down in a candid letter yesterday to President Reagan in which she said her resignation is in "the interests of our country."

Von Damm, a friend and former secretary of Reagan's, became a center of Vienna social gossip early this year by divorcing her third husband and marrying Peter Guertler, owner and manager of the aristocratic Hotel Sacher.

In her letter to Reagan, for whom von Damm has worked since his early days as governor of California, the ambassador suggested that she leave her post at the end of the year. An administration official said he expects the resignation and its timing to be accepted by Reagan.

"You will recall that when I wrote you about my decision to marry Peter I told you I was well aware of the potential consequences," von Damm said in her letter. " . . . For my own part I have always been cognizant of the priority my official duties deserved and I sought to scrupulously adhere to the high standards of public service which you have set for your administration.

"Being at all times aware that public perceptions are as important as reality in the world in which we live and work, however, I must recognize there are voices that continue to assert a conflict of interest between my professional responsibilities and my personal situation," the letter continued. "These circumstances have led me to the conclusion that the interests of our country and your own are best served by your appointment of a new ambassador to Austria."

She added in a postcript that she will always be Reagan's "goodwill ambassador."

Friends of von Damm said she plans to divide her time between New York, Washington and Vienna, possibly helping her husband set up a U.S. hotel.

Von Damm, 45, left Austria in 1959 after marrying an American soldier. She worked for Reagan during his two-term governorship, helped raise funds for his presidential campaigns and served for a time as White House personnel director, where she was credited with increasing the number of female appointees.

A recent evaluation of her ambassadorship in the Austrian newsweekly Profil devoted attention to the gossip but quoted approving comments from several Austrian officials and said she "really sells her country better than her predecessors, often colorless, did."