Fort O'Brien is a public park and historic site in Machiasport, Maine. It is also known as Fort Machias or the Fort O'Brien State Historic Site.

Built in 1775, Fort O'Brien is one of the few forts in the state of Maine that has been around and used during three major wars in the past (American Revolutionary War, War of 1812, and the American Civil War). It is also a frequently destroyed fort that still survives today, as it was first heavily damaged by the British the same year it was built and twice more over a 90 year period.

Visitors of the Fort O'Brien can still find the cannon known as “Napoleon” at the site. Picnic areas are also present in the park.