The letters to Polish-born American philosopher Anna-Teresa Tymieniecka had been kept away from public view in the National Library of Poland for years.
The documents reveal a rarely seen side of the pontiff, who died in 2005.
There is no suggestion the Pope broke his vow of celibacy, the BBC reports.
The friendship began in 1973 when Ms Tymieniecka contacted the future Pope, Cardinal Karol Wojtyla, then Archbishop of Krakow, about a book on philosophy that he had written.
The then 50-year-old travelled from the US to Poland to discuss the work.
Shortly afterwards, the pair began to correspond. At first the cardinal's letters were formal, but as their friendship grew, they become more intimate.
The pair decided to work on an expanded version of the cardinal's book, The Acting Person. They met many times - sometimes with his secretary present, sometimes alone - and corresponded frequently.
In 1974, he wrote that he was re-reading four of Ms Tymieniecka's letters written in one month because they were "so meaningful and deeply personal".
Photographs which have never been seen before by the public reveal Karol Wojtyla at his most relaxed. He invited Ms Tymieniecka to join him on country walks and skiing holidays - she even joined him on a group camping trip. The pictures also show her visiting him at the Vatican.
"Here is one of the handful of transcendentally great figures in public life in the 20th Century, the head of the Catholic Church, in an intense relationship with an attractive woman," says Eamon Duffy, Professor of the History of Christianity at Cambridge University.
In 1976, Cardinal Wojtyla attended a Catholic conference in the US. Ms Tymieniecka invited him to stay with her family at their country home in New England.
She appeared to have revealed intense feelings for him because his letters immediately afterwards suggest a man struggling to make sense of their friendship in Christian terms.
In one, dated September 1976, he writes: "My dear Teresa, I have received all three letters. You write about being torn apart, but I could find no answer to these words."
He describes her as a "gift from God".
The BBC has not seen any of Ms Tymieniecka's letters. It is believed copies of them were included in the archive that was sold to the Polish National Library by Ms Tymieniecka in 2008, six years before she died. But they were not with the Pope's letters when the BBC was shown them. The National Library of Poland has not confirmed that they have Ms Tymieniecka's letters.
See BBC report here