The Penobscot Expedition was a 44-ship American naval armada during the Revolutionary War assembled by the Provincial Congress of the Province of Massachusetts Bay. The flotilla of 19 warships and 25 support vessels sailed from Boston on July 19, 1779 for the upper Penobscot Bay in the District of Maine carrying an expeditionary force of more than 1,000 American colonial marines (not to be confused with the Continental Marines) and militiamen. Also included was a 100-man artillery detachment under the command of Lt. Colonel Paul Revere.

The expedition's goal was to reclaim control of mid-coast Maine from the British who had captured it a month earlier and renamed it New Ireland. It was the largest American naval expedition of the war. The fighting took place on land and at sea around the mouth of the Penobscot and Bagaduce rivers at Castine, Maine, over a period of three weeks in July and August. It resulted in the United States' worst naval defeat until Pearl Harbor 162 years later in 1941.

On June 17, British Army forces landed under the command of General Francis McLean and began to establish a series of fortifications around Fort George on the Majabigwaduce Peninsula in the upper Penobscot Bay, with the goals of establishing a military presence on that part of the coast and establishing the colony of New Ireland. In response, the Province of Massachusetts raised an expedition to drive them out, with some support from the Continental Congress.

The Americans landed troops in late July and attempted to besiege Fort George in actions that were seriously hampered by disagreements over control of the expedition between land forces commander Brigadier General Solomon Lovell and expedition commander Commodore Dudley Saltonstall, who was later dismissed from the Navy for ineptitude. For almost three weeks, General McLean held off the assault until a British relief fleet arrived from New York on August 13 under the command of Sir George Collier, driving the American fleet to destruction up the Penobscot River. The survivors of the expedition made an overland journey back to more populated parts of Massachusetts with minimal food and arms.