San Mateo is an unincorporated community in Putnam County, Florida, United States, located just southeast of the city of Palatka. It is on the east shore of the St. Johns River between buoys 12 and 15; stretching from Buzzard Island to the entrance to Dunn's Creek.San Mateo is located at 29°36′25″N 81°35′06″W / 29.607°N 81.585°W / 29.607; -81.585, with an elevation of 72 feet.