The document will be sold Nov. 14 at Christie's. The only other known copy of the proclamation is at the Library of Congress, the auction house said.
The proclamation, which set the first national day of thanksgiving for Thursday, Nov. 26, 1789, is estimated to sell for $8 million to $12 million. It's being sold by a private American collector.
Washington marked the day by attending services at St. Paul's Chapel in Manhattan.
In 1863, President Abraham Lincoln made Thanksgiving Day a permanent national holiday to be celebrated annually on the fourth Thursday of November.
In 1939, President Franklin D. Roosevelt moved it to the third Thursday in November. Two years later, Congress passed a resolution establishing the fourth Thursday in November as Thanksgiving Day.
Washington's proclamation says in part: "Both Houses of Congress have by their joint Committee requested me to recommend to the People of the United States a day of public thanksgiving and prayer . Now therefore do I recommend and assign Thursday the 26th day of November next to be devoted by the People of these States to the service of that great and glorious Being."