The fold-out note thanks his Aunt Harriet for presents she gave him, including a book about ships.
He sent the thank you message shortly after Christmas in 1951 to his mother Julia's younger sister.
During his Beatles career, Lennon often wrote to friends and family but this is understood to be his first letter.
In the letter, which has a few grammatical errors, the young Lennon says how much he is enjoying reading a book about famous ships.
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"The book that you sent to me is a very interesting one. I am at the bottom of page 18 at the moment. The story is famous Ships its all about a man called Captain kidd the pirate.
"I am on the second chapter, the first chapter is called the Victory and the second is called the Mary Celeste.
"Thankyou for the red jumper that you sent to me.
The letter - which was formerly the property of his cousin David Birch, Harriet's son - is to be sold at an online auction that closes on April 3, the BBC reported.
The auction contains around 300 lots of Beatles and music memorabilia, including the address book from Liverpool's Cavern Club where the group played many gigs, and a collection of records that were previously owned by Lennon.
Last month, a lock of hair snipped from the head of Lennon 50 years ago as he prepared for a film role was bought by a British collector for USD 35,000.