Portsmouth Harbor Lighthouse is a historic lighthouse located within Fort Constitution in New Castle, New Hampshire, United States.

The station was established in 1771 and was the 10th of 11 light stations established prior to the American Revolution. The first tower was a shingled wooden structure with an iron lantern and copper roof. Its light source was three copper oil lamps.

The first tower was replaced in 1804 by an 80-foot (24 m) octagonal wooden tower approximately 100 yards (91 m) east of the 1771 tower. In 1851, twenty years after the establishment of Whaleback Lighthouse, the tower was shortened to 55 feet (17 m). Three years later, in 1854, the tower was fitted with a Fourth (4th) Order Fresnel lens.

In 1878, a new 48-foot (15 m) cast-iron, brick-lined lighthouse tower was erected on the same foundation as the 1804 tower. When the new tower was completed, the surrounding remains of the 1804 tower were removed.

The current light is a fixed green signal that is visible for 12 nautical miles (22 km; 14 mi). The light is made green by an acrylic cylinder that surrounds the lens.

Other structures at the light station that are still standing are the 1903 oil house (restored in 2004) and the 1872 keeper's house (currently United States Coast Guard offices).

The lighthouse was added to National Register of Historic Places in 2009.

Founded in 2001, the Friends of Portsmouth Harbor Lighthouses are a chapter of the American Lighthouse Foundation (ALF). The mission of the Friends is to work for the preservation of Portsmouth Harbor Lighthouse and associated structures, as well as to gather and preserve the history of the important historic site and to share these resources with the public. The United States Coast Guard owns Portsmouth Harbor Lighthouse and still maintains the active aids to navigation equipment. The group is licensed through ALF to care for the tower, oil house, and wooden walkway. As of November 2008, the Friends are also responsible for Whaleback Lighthouse, and altered the chapter name to reflect this.

• National Register of Historic Places listings in Rockingham County, New Hampshire

• Friends of Portsmouth Harbor Lighthouses
• American Lighthouse Foundation
• SCARED! at the Portsmouth Harbor Lighthouse