South Wales Police received a report of a serious road traffic collision, in the westbound carriageway of the M4 motorway, near junction 47 at Penllergaer.
The collision, which occurred at approximately 01.20am on Monday, July 2 involved two vehicles.
One man has died at the scene.
The male driver of the second car has been conveyed to Morriston Hospital, with injuries that are described as serious, but not life threatening.
The eastbound carriageway was closed for approximately three and a half hours, but has now fully reopened.
The westbound carriageway remains closed at this time, to allow for investigation and recovery work to be completed, but is expected to re-open shortly.
South Wales Police thanks the local community for their continued patience and understanding during this time.
A spokesperson said: "Anyone who witnessed the collision, or the manner in which the vehicles were being driven prior to the collision, is asked to contact the Serious Collision Investigation unit via 101 quoting reference number 242267."
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