The 700-year-old parchment letter will be displayed at National Records of Scotland on September 29.
It gives a “tantalising glimpse” into the life of the man who led Scottish forces through the First War of Independence.
The document was last seen in 2014 and can be rarely shown given the strictly controlled conditions required to protect the letter.
Dr Tristram Clarke, NRS archivist, said: “Written in November 1300, the document offers a tantalising glimpse into a mysterious period of Wallace’s career.
“King Philip IV of France wrote to his agents in Rome, commanding them to ask Pope Boniface VIII to view William Wallace favourably.
“The letter proves Wallace played a role in the turbulent politics of medieval Europe, when the Pope was trying to encourage peace between England and France, and to challenge England’s claim to control Scotland.”
Wallace was eventually betrayed, captured by the English and executed in London in 1305.
King Philip’s letter was discovered in the Tower of London in the 1830s.
In 2011 a panel of expert historians and archivists concluded it was likely to have been in Wallace’s possession when he was captured.
It is currently on loan to the National Records of Scotland from The National Archives in London.
The public will be able to see the letter at New Register House, 3 West Register Street, as part of Doors Open Day.