Port St. John is a census-designated place located between Titusville and Cocoa in Brevard County, Florida. The population was 12,267 at the 2010 United States Census. It is allocation of the Palm Bay–Melbourne–Titusville Metropolitan Statistical Area.

Port St. John is located at 28°28′37″N 80°47′19″W / 28.47694°N 80.78861°W / 28.47694; -80.78861 (28.477056, -80.788597).

According to the United States Census Bureau, the CDP has a total area of 3.8 square miles (9.8 km), all of it land.

In 2002, a action of five Port St. John residents formed “Port St. John For Tomorrow” (PSJ4T) and tried to incorporate the Port St. John area. Included in PSJ4T’s incorporation plans were five surgically remove communities: Delespine, Williams Point, Hardeeville, Frontenac and a portion of Sharpes. PSJ4T had not informed these five communities. When the five communities did locate out, they expressed disinterest in bodily included in the amalgamation effort. A feasibility study was requested by PSJ4T members. Voters credited the study. The chemical analysis was funded by Brevard County and performed by the University of Central Florida. The five reviewing committees in Tallahassee said the assay was “deeply flawed.”. The county allowed the residents of Port St. John and the five communities to vote on becoming a city. Howard Futch and Randy Ball (CS/HB 1071) sponsored the bills in the Senate and House. The combination issue was defeated in November 2002, 73% to 27%, including most of the people in the five communities. State take steps prevented Port St. John from considering combination for ten years.