The postcard was written on the Titanic by first-class passenger Archibald Gracie and is, among other things, postmarked Queenstown in southern Ireland, one of two harbors that the Titanic called at before its fateful Atlantic voyage in April 1912.
It was sent to a relative of the person who has now left it to the auction house Henry Aldridge and Son in the British town of Devizes.
"It's a good ship, but I'll wait with my final judgment about her until the trip is over", writes Gracie on the Titanic's special postcard from the shipping company White Star Line.
The letter was bought for 300,000 pounds, 3.9 million Swedish kronor, by an American collector – about five times the expected price.
The letter is believed to be the only remaining one of those that Gracie wrote on board the Titanic, which sank after colliding with an iceberg near Newfoundland in Canada. 1,500 people went down with the ship, Gracie ended up in the icy water but survived and later wrote a book about his experiences on board.
The book "The Truth about Titanic", which was also published in Swedish, is considered by experts to be one of the best eyewitness accounts of life on board the ship.