Port Allen Harbor is located approximately 20 miles south of the town of Līhue, Kauai.  Port Allen Harbor was authorized by the Rivers and Harbors Acts of 1935 and 1945. The local sponsor is the Harbors Division, Department of Transportation, State of Hawaii.  The project is in the operations and maintenance phase.

The harbor was cost shared with the local sponsor and completed by the U.S. Army Corps of Engineers in 1948. The project consists of a 1,126-foot-long breakwater, a 35-foot-deep basin (1500 feet long, and 1200 feet wide), and an entrance channel 500 feet wide. 

In January 1984, repairs were made to the breakwater head damaged that was damaged by Hurricane Iwa in November 1982. In July 1991, repairs were again made to the breakwater head damaged by heavy storms in 1989. In 1994, additional repairs were made to areas damaged by Hurricane Iniki in September 1992. In February 2009, repairs were made to the 280-foot-long breakwater root section, which was damaged as the result of wave action sustained over numerous years.

Port Allen Harbor is one of two commercial harbors on the island of Kauai.  The harbor is particularly important for commercial charter vessels, liquid bulk cargo and military vessels servicing the Navy’s Pacific Missile Range Facility.

The U.S. Army Corps of Engineers is responsible for maintaining the breakwater structure and project depths in the federal limits. Maintenance dredging was conducted in 1999 and 2021.