The key, which belonged to 23-year-old steward Sidney Sedunary, is to go on sale at Aldridge & Son in Wiltshire
A locker key from the Titanic is expected to be sold for up to 50,000 at auction.
The key was used by 23-year-old Sidney Sedunary, from Shirley, Southampton, who was second third-class steward on the passenger liner and one of 1,500 people who died when the vessel hit an iceberg in April 1912 in the north Atlantic.
His body was recovered, and his possessions, including his pocket watch and the exceptionally rare key, which held a brass tag with the words Locker 14 F Deck, were sent to his pregnant wife, Madge.
Some of Sedunarys belongings were donated to the Southampton Maritime Museum by his son, also named Sidney, but many remained with the family, including the key.
It will be sold on 22 October by the auctioneers Henry Aldridge & Son, of Devizes, Wiltshire, who are specialists in Titanic memorabilia. The firms consultant, Andrew Aldridge, said: It is the first time it has ever been offered for auction.
He said that the significance of the key was linked to evidence given in May 1912 to the British inquiry into the liners sinking, by John Hart, a steward who had been on board. Aldridge said the evidence showed Sedunary [was] heroically working on F Deck on the very night of the sinking, even though it was flooded.
Sedunarys son gave the key to a family member visiting from Australia in 1981.