In the note, written to the seller's great-uncle on April 10, 1912, first-class passenger Archibald Gracie wrote of the ill-fated steamship: "It is a fine ship but I shall await my journeys end before I pass judgment on her."
The letter was sold to a private collector from the United States on Saturday, according to auction house Henry Aldridge & Son in Wiltshire, England.
The hammer price of STG300,000 ($A623,000) far exceeded the initial estimate of STG60,000 ($A125,000).
The letter is believed to be the sole example in existence from Gracie from onboard the Titanic, which sank off Newfoundland on April 14, 1912, after hitting an iceberg, killing about 1500 people on its maiden voyage.
Auctioneer Andrew Aldridge described it as an "exceptional museum-grade piece".
Gracie, who jumped from the ship and managed to scramble onto an overturned collapsible boat, was rescued by other passengers onboard a lifeboat and was taken to the RMS Carpathia.
He went on to write The Truth About The Titanic, an account of his experiences, when he returned to New York City.
Gracie boarded the Titanic in Southampton on April 10, 1912, and was assigned first-class cabin C51.
His book is seen as one of the most detailed accounts of the events of the night the ship sank, Aldridge said.
Gracie did not fully recover from the hypothermia he suffered, and died of complications from diabetes in late 1912.
The letter was postmarked Queenstown, Ireland, one of two stops the Titanic made before sinking.
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