The letter was penned by ship steward Richard Geddes to his wife and describes how the Titanic almost collided with another vessel days before the disaster on April 14 1912.
It Mr Geddes says: "My dearest Sal, We got away yesterday after a lot of trouble.
“As we were passing the New York and Oceanic the New York broke her ropes and very nearly ran into us, but we just happened to avoid a collision.
“I could see visions of Belfast it must have been a trying time for the Captain."
Mr Geddes was one of more than 1,500 passengers and crew who died in the disaster.
It was sent to his wife in Southampton in a White Star Line envelope.
The letter is being sold at auction and is estimated that it will fetch up to £18,000 when it appears in Devizes, Wiltshire, on April 27.
Auctioneer Andrew Aldridge said the letter was "exceptional" adding: “First and foremost it was written onboard the Titanic, it has its envelope, the lot also contains official paperwork relating to Mr Geddes and finally the content is superb, describing the near miss that Titanic nearly suffered that would have changed history.”
It is being sold alongside the death certificate of Mr Geddes which acknowledges he died during the disaster as well as several pictures of him and his wife.
In the letter Mr Geddes told his wife he believed the Titanic would be “a great deal better” and “steadier” than other ships.
“If we get in on time on Wednesday and there happens to be a boat I will write from New York,” he wrote.
He signed it “with fondest love and kisses to my dear wife and kiddies”.