圣约翰斯县（St. Johns County, Florida）是美国佛罗里达州东北部的一个县，东临大西洋。面积2,127平方公里。根据美国2000年人口普查，共有人口123,135。县治圣奥古斯丁（St. Augustine）。

成立于1821年7月21日，是该州最早的两个县之一（另一个是艾斯康比亚县，当时以萨旺尼河 Suwannee River 为界）。县名纪念施洗圣约翰。

St. Johns County is a county of the U.S. state of Florida. As of the 2010 United States Census, the county's population was 190,039. The county seat and largest incorporated city is St. Augustine.

The county was established in 1821. It is one of the two original counties established after Florida was ceded to the United States, at the start of the Florida Territorial period, and corresponded roughly with the former colonial province of East Florida. It was named for the St. Johns River, which runs along its western border.