A 27 year old woman was found laying in the carriageway of the M4 motorway between junctions 47 and 48 yesterday morning.
Police have named her as Abigail Hussey from Penllergaer.
Emergency services attended but despite immediate medical assistance she was declared deceased.
South Wales Police is appealing for any witnesses who may have travelled along the A48 Swansea Road, Penllergaer in the vicinity of the motorway overbridge where the female is known to have been, who may have seen her at the location.
Motorists who were travelling along the motorway and may have seen this female on the overbridge, or a gold coloured Citroen C4 at this time, and may have dash cam footage are also asked to contact South Wales Police on 101 quoting reference 2000163094.
Family members are currently being supported by a specially trained police officer.