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 Wainwright Society, formed after his death in 1991, has chosen Fix the Fells as its main beneficiary for 2017.
Up to Â£20,000 will be used to repair badly eroded Long Stile, the final ascent on to High Street summit between Ullswater and Haweswater.
A Fix the Fells spokesman said the group was "delighted" to be chosen.
Derek Cockell, secretary of The Wainwright Society, said: "Next year is the 60th anniversary of Wainwright's Far Eastern Fells book, which came out in 1957.
"We approached Fix the Fells and asked them if they had any paths that they were going to repair in that area and they told us about Long Stile which of course Wainwright pioneered as a wonderful route up High Street."
Fix the Fells, which repairs eroded and damaged footpaths in the Lake District, is expected to complete the work in 2018 with the help of National Trust rangers and Lake District National Park Authority volunteers.
Mr Wainwright, who was born in 1907, was a significant figure for Cumbria, compiling drawings of 214 fells over more than 10 years of exploration in the 1950s and 60s.
The Wainwright Society was formed to keep alive the things he promoted through his guidebooks.
The earliest known letter written by John Lennon is expected to fetch more than £30,000 at auction next month.
[NEXT_CONCEPT]
A Lake District path expert fell walker Alfred Wainwright described as the "connoisseur's route" is to be repaired thanks to a charity set up in his name.