She had been in a hidden annexe for two months, hiding from the Nazi occupiers of Amsterdam with her family and another Jewish family.
It is not known why she covered up the two pages with brown paper, but several times in her diaries she voices fears that others might pry into her writings.
The pages contain four "dirty jokes" and an explanation of sex, contraception and prostitution, the ABC reports, which were covered with gummed pieces of brown paper when the diary was first found - presumably Anne's attempt to hide her risqué writing from her family.
The Anne Frank House, a museum located in Frank's former hiding place, did not quote directly from the text it had recovered.
Peter de Bruijn, one of the partners in the diary research and a senior researcher at the Huygens Institute for the History of the Netherlands, said the uncovered text is significant because it shows Frank's first attempt at writing in a more literary tone.
He said the jokes "make it clear that Anne, with all her gifts, was above all also an ordinary girl".
The pages, dated to September 28, 1942, were contained in the red-and-white checkered diary Anne had received for her birthday in June of that year, shortly before they went into hiding.
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She listed 4 dirty jokes, and added 33 lines about sex education and prostitutes.
"I sometimes imagine that someone might come to me and ask me to inform him about sexual matters", Frank wrote.
The posthumously published diary of Anne Frank is world famous.
But exactly when and exactly why Anne blocked out the pages will likely never be known.
Researchers were at pains to point out that it is not the first time that Frank wrote about sex in her diary.
Her father Otto was the only member of the family to survive the war.
The pages were first discovered in 2016 in her first diary with a red-checked cover. Though Anne herself edited her diary with an eye to publication, the book-released eight years after her death from typhus in theBergen-Belsen concentration camp at age 15-contained additional cuts. It is now considered one of the most important documents to have emerged from the Holocaust, and has been read by millions of people and translated into 60 languages.