The fold-out note, penned by Lennon when he was 11 years old, 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.
He wrote: "Dear Harrie Thankyou for the book that you sent to me for Christmas and for the towel with my name on it, And I think it is the best towl (sic) I've ever seen.
"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.
"I hope you have a happy new year. Love from John x"
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 3 April.
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.
The earliest known letter written by John Lennon is expected to fetch more than £30,000 at auction next month.