The great fire of Portland, Maine, sometimes known as the 1866 great fire of Portland, occurred on July 4, 1866—the second Independence Day after the end of the American Civil War. Five years before the Great Chicago Fire, this was the greatest fire yet seen in an American city. It started in a boat house on Commercial Street, likely caused by a firecracker or a cigar ash. The fire spread to a lumber yard and on to a sugar house, then spread across the city, eventually burning out on Munjoy Hill in the city's east end.

Two people died in the fire and 10,000 people were made homeless. 1,800 buildings were burned to the ground. This included the federal Exchange Building by which was replaced with the custom house. Soon after the fire, Portland native and acclaimed poet Henry Wadsworth Longfellow described his old home town: "Desolation, desolation, desolation. It reminded me of Pompeii, that 'sepult city."

More than 600 buildings were constructed in four months after the fire.

• History of Portland, Maine (1886)
• Summers, Lydia B., ed. (1999). Portland. Greater Portland Landmarks.
• Bufford, J. H. (1866). "Great fire of Portland 1866". Maine Memory Network. Maine Historical Society. Archived from the original on Jul 10, 2011. Retrieved 2009-07-22.
• Horton, Linda (June 16, 2008). "Portland, ME Fire, Jul 1866 - Terrible Conflagration". GenDisasters. Archived from the original on 2012-02-26. Retrieved 2009-07-22.

43°39′5″N 70°15′25″W / 43.65139°N 70.25694°W / 43.65139; -70.25694