Riddoch Highway is a rural highway in south-eastern South Australia, designated as route A66 between Keith and Mount Gambier, with the remainder between Mount Gambier and Port MacDonnell designated as route B66. It is named after John Riddoch, the first white settler landholder and vigneron in Coonawarra.

Route

The Riddoch Highway branches from the Dukes Highway at Keith and travels south through Padthaway, Naracoorte, Penola, Nangwarry, Tarpeena, and Mount Gambier to Port MacDonnell and nearby Cape Northumberland. It passes through grazing and cereal-growing land, horticultural and vineyards (within the following wine regions – Padthaway, Wrattonbully, Coonawarra and Mount Gambier), and plantation timber, predominantly pinus radiata.

Safety

The Royal Automobile Association of South Australia has rated the highway at 5/10.

Major intersections