Route 109 is a highway in St. Louis County, Missouri. Its northern terminus is at Route CC Chesterfield; its southern terminus is at Routes W and FF in Jefferson County. The road continues north as Eatherton Road, curves around Spirit of St. Louis Airport and connects with Chesterfield Airport Road, providing a "backway" to the Chesterfield area.

The original alignment of Route 109 was once Route C and the original route was renamed Route BA (the only such designation in the state). At Eureka, Route 109 crosses under Interstate 44 and continues into Jefferson County until it reaches the intersecion of Routes W and FF.