State Road 166 (SR 166) is a 5.054 miles (8.134 km) long state road in Jackson County, Florida, running from US 90/SR 73 in Marianna, Florida to SR 71. The road is almost entirely two lanes wide, except in portions where third center left turn lanes appear.Despite ending in an even number, the road runs mostly north and south in and north of Marianna, Florida. While most of the road is signed as a north-south road, the northernmost portion is signed as an east-west road.