圣约翰河（英语：St. Johns River）是美国佛罗里达州境内最长的河流，以发展商业和休闲娱乐产业为重。它长达499公里（310英里），河幅宽跨12个县，其中包含佛州最大的3个县别。圣约翰河河源至入海口的垂直海拔差约低于9.1米（30英尺）；和多数佛州的水路一样，流速缓，约每小时0.2公里（⅓英里），被当地人称为「懒惰河」。它是美国少数流向自南向北的河川。圣约翰河水道范围形成许多湖泊，或汇流至河中，但河道最宽处只有4.8公里（3.0英里），横跨帕拉特卡和杰克逊维尔，后者更是圣约翰河畔最大的都市区。它的最窄处位于源头，为印第安河县内一个无法航行的沼泽湿地。圣约翰河流域内总计约有3500万人栖居。

The St. Johns River (Spanish:Río de San Juan) is the longest river in the U.S. state of Florida and its most significant for commercial and recreational use. At 310 miles (500 km) long, it winds through or borders twelve counties, three of which are the state's largest. The drop in elevation from headwaters to mouth is less than 30 feet (9 m); like most Florida waterways, the St. Johns has a very low flow rate 0.3 mph (0.13 m/s) and is often described as "lazy". It is believed to be one of the few rivers that flow north, although there are hundreds of the kind worldwide. Numerous lakes are formed by the river or flow into it, but as a river its widest point is nearly 3 miles (5 km) across. The narrowest point is in the headwaters, an unnavigable marsh in Indian River County. The St. Johns drainage basin of 8,840 square miles (22,900 km) includes some of Florida's major wetlands. It is separated into three major basins and two associated watersheds for Lake George and the Ocklawaha River, all managed by the St. Johns River Water Management District.