Southern Expressway is an 18.5 km (11.5 mi) freeway through the southern suburbs of Adelaide, South Australia. It is the southern part of the North–South Corridor which extends the full length of Adelaide and is being built to urban freeway standard. It is designated part of route M2.

It was built as a corridor to relieve heavy traffic from the major arterial, Main South Road in Adelaide's south, originally as a reversible one way freeway, and was the world's longest after its completion in 2001. Between 2010 and 2014, it was duplicated and it is now a standard dual-carriageway freeway.

Construction of the expressway included the Adelaide Southern Veloway for cyclists, which runs alongside it for 7 km, from Marion/Main South Road to the Reynella Interchange. South of the Reynella Interchange, the bikeway becomes a shared track for the remaining 12 km of the expressway. The expressway crosses over the Coast to Vines Rail Trail at both Panalatinga Road and at the expressway's southern end.

Southern Expressway starts in the central median of South Road in St Marys, and continues southwest in a lowered trench between the carriageways of South Road, until it crosses the Sturt River. It continues in a south-westerly direction in its own alignment, running roughly parallel to Main South Road through Reynella to eventually terminate at an intersection with it just north of Old Noarlunga.

The expressway is the southern part of a north–south freeway originally conceptualised under the Metropolitan Adelaide Transport Study (MATS), completed in 1965, as a freeway bypassing the city from Dry Creek to Old Noarlunga. The MATS plan proved unpopular, and in 1971 all further highway construction in Adelaide was postponed for a period of ten years. In 1983 plans for a freeway north of Darlington were abandoned, and the land that had been set aside was progressively sold off.

In 1984 the state government announced plans to develop a "third arterial road" for the south. In 1987 the project was split into two phases: the first was the upgrading of Main South Road and Marion Road in the Darlington area, and the second was a new road from Darlington to Reynella. Phase one was completed in 1994 with the widening of Main South Road to eight lanes between Ayliffes Road and Seacombe Road, and the widening of Marion Road to six lanes between Main South Road and Sturt Road.

Phase 2 became the expressway, which was constructed in two stages: from Darlington to Reynella, and from Reynella to Old Noarlunga. It was developed as a one-way reversible road, with future duplication provided for in the roadworks base, to be constructed when the need arose. It used the remaining "Noarlunga Freeway" reservation, except for the northernmost kilometre, where a new route through O'Halloran Hill was chosen to provide high-quality links with Marion and Main South Roads without interfering with existing infrastructure at Sturt.

Construction of the expressway commenced in July 1995 and the first stage was opened to traffic on 17 December 1997. Construction commenced on the second stage in February 1999 and was opened to traffic on 9 September 2001. The total cost of the reversible freeway was A$76.5m.^{[citation needed]}