Arkansas Highway 121 is a designation for two state highways in Lee County, Arkansas. One route of 9.49 miles (15.27 km) runs from the St. Francis River to US Route 79 (US 79) northeast of Marianna. A second routing begins north of Marianna at Highway 1 and runs essentially in a half-loop counterclockwise to LaGrange.

Lee County is a county located in the U.S. state of Arkansas. With its eastern border formed by the Mississippi River, it is considered to be part of the Arkansas Delta. As of the 2020 census, the population was 8,600. The county seat is Marianna.