First National Bank of Port Jefferson is a historic bank building located at Port Jefferson in Suffolk County, New York. It is a 2+1⁄2-story brick building with a 2-story turret that projects from the second story of the southwest corner and rises above the roof line. The turret has a steeply pitched, cone shaped slate roof and three windows on both levels. The main entrance to the building used to be directly below the turret, but was made into a window around 1949 when a 1-story brick, flat-roofed addition was built on the east side of the bank building. In 1948, the building ceased being used for a bank and was purchased by the Town of Brookhaven, who completed the 1-story addition the following year.