Border Field State Park, State park in Imperial Beach, California

Border Field State Park covers beach and coastal habitat on the Mexico–United States border within Imperial Beach, San Diego County. It forms the southern part of the Tijuana River National Estuarine Research Reserve, with salt and freshwater marshes. The park contains the southernmost point in California and Friendship Park just inland from where the border meets the ocean.

The park marks where surveyors set the initial boundary point between the United States and Mexico following the 1848 Treaty of Guadalupe Hidalgo. This location documents when the international border was first established.

Friendship Park serves as a meeting spot where families from both sides of the border gather to connect and talk with each other. The place holds special meaning for local communities as a point of contact between two nations.

Border Field State Park was closed to the public in 2020 during the COVID-19 outbreak and stayed shut through January 2025, with flooding, sewage issues and new border fence construction cited. Check current access before travelling; the entrance lies at the western end of Monument Road in Imperial Beach.

The park's southern edge is the Initial Point of Boundary Between U.S. and Mexico, where the joint boundary survey began after the 1848 Treaty of Guadalupe Hidalgo. During World War II the site served as Border Naval Outlying Landing Field.