The letter, written in 1788 when Adams and her husband were about to return from London, is to her uncle, Dr. Cotton Tufts of Weymouth, Mass., The Boston Globe reported. Lawrence Perera said the letter was in a manila envelope in a desk with family documents that had belonged to his mother.
"It's very, very lucky that we found it at all," he said.
Perera, who found the letter in January, has donated it to the Massachusetts Historical Society, which announced its acquisition Thursday.
Margaret A. Hogan, a managing editor at the historical society, said Adams was a prolific letter writer and about 2,500 are known, 100 of them to Tufts, who often looked after the family's business affairs when John and Abigail were away. But she said previously unknown letters are unusual, coming to light about once every five years.
Adams, writing on March 2, 1788, at the end of four years in Paris and London, asks Tufts "to pay the freight" for a shipment of sherry, saying she hopes to drink some of it with him. She also talks about European politics, the lack of respect for the new U.S. government and domestic difficulties.
"I find it the most troublesome removal I have ever made,'' she said.