A common dolphin was found stranded on the beach in Mumbles on Friday.
The female was discovered near the lifeboat station after washing up during the morning's high tide.
Passersby spotted the dolphin on the pebbles just before lunchtime.
William Kay, a marine biologist at Swansea University and part of the RNLI crew, was at the scene for the recovery of the dolphin from the beach.
He said: "It was a female common dolphin (Delphinus delphis).
"It washed up on this morning's high tide. We found it around 11.30am and I was on scene by 12.30pm.
"It was on the beach so we moved it to the lifeboat slipway to make sure the tide didn't wash it away again.
"A representative from the Cetacean Strandings Investigation Programme (CSIP) then came down to take it away for post-mortem examination.
"We are not sure yet what it died of and where it was when it died but we suspect it was local as it washed up here and was quite fresh."
The common dolphin species are medium-sized and adults can range between six foot two inches and eight foot two inches long, and can weigh between 80 to 235kg.
Their backs are dark and the stomach is white, while on each side is an hourglass pattern which is coloured light grey, yellow or gold in front and dirty grey in the back.
A spokesman for the Whale and Dolphin Conservation (WDC) said: "It is very hard to determine cause of death and that may never be known unless a necropsy is carried out soon after death.
"Sadly there are many man made threats that may cause a dolphin like this one to wash up.
"Noise from underwater surveys for oil/gas, or from military exercises can cause strandings, so can accidental capture in fishing nets or gear.
"Plenty of others of this species wash up at this time of year due to fishing."