BRUNSWICK — One of the most prestigious medals earned by one of Maine’s most decorated sons was discovered at a church sale and turned over to a Brunswick-based organization for safe keeping, the group announced Monday.

The U.S. Army Medal of Honor was awarded to Civil War hero Joshua Chamberlain — who would go on to become president of Bowdoin College and governor of Maine — in 1893 for “distinguished gallantry” in the Battle of Gettysburg 30 years earlier.

The artifact was given to the Pejepscot Historical Society, which owns the Joshua L. Chamberlain Museum in Brunswick, by a donor who wishes to remain anonymous, the organization announced Monday afternoon. The individual who came to own the medal found it in the back of a book he had purchased “several years ago” at a sale held by First Parish Church in Duxbury, Mass., according to the society.

Chamberlain’s last surviving descendant, granddaughter Rosamond Allen, left her estate to that church upon her death 13 years ago.

“Though it seems almost too good to be true, we are confident that we are now in possession of Joshua Chamberlain’s original Medal of Honor,” said Pejepscot Historical Society Director Jennifer Blanchard in a Monday statement. “All of the experts we’ve consulted believe it to be authentic, and we are tremendously honored to return the medal to Chamberlain’s home in Brunswick. The timing couldn’t be better, since the medal was awarded for Chamberlain’s distinguished service in the Battle of Gettysburg, whose 150th anniversary we mark in 2013. Our gratitude to the donor who discovered this treasure, and knew its importance to us and to the state of Maine, knows no bounds.”

Chamberlain was a Brewer native who later attended Bowdoin College and led the Union Army’s 20th Maine Infantry Regiment in its crucial defense of the Little Round Top high-ground at Gettysburg, a stand considered by many historians to be one of the defining moments of the Civil War.

Bowdoin College has in its possession a 1904 Army Medal of Honor that belonged to Chamberlain, representing an updated design of the award authorized by Congress at the time, according to the Pejepscot Historical Society.

The Legislature allowed recipients to keep their previous versions of the medal as long as they did not display both at the same time, and the 1893 artifact now in the society’s collection represents the earlier award.

Blanchard worked with experts at the Maine State Museum, the Library of Congress, the Smithsonian, and the Department of the Army’s Awards and Decorations Branch to confirm the medal’s authenticity, according to the society’s Monday announcement.

“Based upon the documentation submitted and the historical documentation available to this office, we are able to confirm that [the] Pejepscot Historical Society medallion is the 1862 United States Army Medal of Honor design,” said U.S. Army Lt. Col. Michael A. Ries, assistant chief of the Awards and Decorations Branch, in a statement. “It is an honor to authenticate the Medal of Honor bestowed upon Colonel Chamberlain for his extraordinary heroism on July 2, 1863.”

Historians throughout Maine lauded the find and donation. Maine State Museum Director Bernard Fishman in a statement called it “one of the most interesting discovery and donation stories on record.”

“This is a very exciting discovery since it is the nation’s highest military honor and the original version of the medal that Chamberlain received from the government,” said Maine Department of Conservation historian Tom Desjardin in a statement. “We can only imagine how he felt when he opened the package to see this honor for the first time.”

U.S. Sen. Angus King, a Chamberlain enthusiast who lives in the same Brunswick neighborhood as the Chamberlain Museum, said in a statement the emergence of the medal “undoubtedly captures the hearts and minds of all Mainers.

“This medal is a symbol of their courage and sacrifice, and I am eternally grateful to the donor whose remarkable generosity will now allow Mainers and visitors alike to appreciate it for generations to come,” he said.

The historical society Monday claimed it will soon announce opportunities for the public to see the medal on display.