The relationship between Anna-Teresa Tymieniecka and Pope John Paul II lasted more than 30 years.
The letters to Polish-born American philosopher, Tymieniecka, had been kept away from public view in the National Library of Poland for years.
The documents revealed a rarely seen side of the pontiff, who died in 2005.
There is no suggestion the Pope broke his vow of celibacy.
His Emotional Life
Pope John Paul II was one of the most influential figures of the 20th Century, revered by millions and made a saint in record time.
Nine years after he died, the BBC saw letters he wrote to a married woman, Tymieniecka, that shed new light on his emotional life.
Mrs Tymieniecka was a great hoarder and she seems to have kept everything relating to her 32-year friendship with Saint John Paul.
The friendship began in 1973 when Mrs 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.
Image copyright Photograph provided by Bill and Jadwiga Smith Image captioned Tymieniecka at the time she met Cardinal Wojtyla.
Shortly afterwards, the pair began to correspond. At first, the Cardinal’s letters were formal, but as their friendship grew, they became 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 Mrs Tymieniecka’s letters written in one month because they were “so meaningful and deeply personal.”
Photographs which had never been seen before by the public revealed Karol Wojtyla at his most relaxed.
He invited Mrs Tymieniecka to join him on country walks and skiing holidays – she even joined him on a group camping trip. The pictures also showed her visiting him at the Vatican.