It protects the A174 route between Whitby and Sandsend in North Yorkshire.
Worn-out defences have been replaced along a 0.6 mile (1km) stretch of the road, a popular tourist route, where it runs close to the shoreline.
Several landslips had led to costly repairs over the years and road closures, North Yorkshire County Council said.
More on this story and others from North Yorkshire
A £9m sea defence scheme along a coast road with a history of landslips is to be officially opened later.