The hand-written letters addressed to Swedish aristocrat Gunilla von Post were written on official United States Senate letterhead and were sent shortly after Kennedy's marriage to Jacqueline Bouvier.
The hand-written letters addressed to Swedish aristocrat Gunilla von Post were written on official United States Senate letterhead and are a part of RR Auction's Fine Autographs and Artifacts sale.
The letters are expected to fetch more than $30,000 by the time the auction is completed on May 12.
Kennedy first met von Post while holidaying in the French Riviera in the summer of 1953, just weeks before his marriage to Jacqueline Bouvier.
Kennedy was a US Senator at the time and exchanged letters with the Swedish aristocrat on several occasions throughout the 1950s.
The set of letters was found among von Post's possessions after her death in 2011 and documents the short-lived love affair between her and the future President of the United States.
"Dear Gunilla, I must say you looked well and happy in the photograph you sent me at the Regatta," Kennedy wrote in a letter in 1955.
"It now appears that Congress will not finish until the 5th of August—so I sail on the ‘United States’ which arrives the 10th of August at Le Havre—and I shall be in Sweden on the 12th. Where do I go. Send me your address at Bastaad where you shall be."
In a second letter addressed to von Post in 1956, Kennedy confessed that he was "anxious" to see the Swedish aristocrat.
"Perhaps at first it shall be a little difficult as we shall be strangers—but not strangers—and I am sure it will all work and I shall think that though it is a long way to Gunilla—it is worth it," Kennedy wrote.
In a longer letter from 1956, Kennedy addressed rumors that von Post was to be married.
"If you don't marry come over as I should like to see you. I had a wonderful time last summer with you. It is a bright memory of my life—you are wonderful and I miss you."
Von Post married wealthy Swedish landowner Anders Ekman in 1956 and later detailed her relationship with Kennedy in her 1997 memoir, "Love, Jack".
She said that the pair sent letters back and forth across the Atlantic over a three-year period in the 1950s and claimed that Kennedy was even planning to leave Jackie and start a new life with her.
She wrote that the two spent a week together in August 1955 and later met by chance at the Waldorf Astoria New York in 1958 while she was pregnant with her first child.
Von Post's husband died in a tragic accident in 1960 and she later married Weisner Miller, an American IBM executive.
She died in Florida in 2011 and her letters were previously auctioned for $15,000 in 2015.