Experts who have identified the species of tuna beached at Burry Port last week say it is only the third record of that type of fish being found in UK waters.
The tuna has been examined by experts at Swansea University
The tuna was recovered by three teenagers from a mud bank near the harbour town last Thursday.
Marine biologists at Swansea University say it was a bigeye tuna - usually found warmer waters in open oceans.
Weighing 83.5lb (38kg) and measuring 1.34m in length they said it was a "very unusual find."
The tuna, which was put into deep freeze at a Swansea fish firm after being brought ashore, has since been passed on to the university's department of biological sciences.
Professor Graeme Hays said the discovery of the fish came just a few weeks after reports of another bigeye tuna caught 70 miles off Land's End by Penzance-based fishermen.
He said the Burry Port tuna was only the third recorded bigeye in British waters.
"This bigeye tuna is a very unusual find in Welsh waters - we will now be carrying out various analyses," he said.
"However, if sea temperatures continue to rise around Wales and the rest of the UK, we may see more and more species such as this appearing in our waters."
He said Wales had also recently attracted other long-distance marine visitors.
The fish was pulled from a mud bank by three teenagers
Last month the department took delivery of a greater amberjack which was caught by fisherman Lee Lavender off Milford Haven.
The 530g fish, usually found in warmer waters such as the Mediterranean or Gulf of Biscay, was found to be one of only five to be recorded in British waters.
Dr John Lancaster, who is a fish recorder for the Welsh Federation of Sea Anglers, said: "Incidences of this type of fish being found in Welsh waters is most definitely increasing.
"We would encourage any members of the public who find unusual fish in Welsh waters to please contact the university.
"Any unusual fish found will be identified and recorded by the team and details are then passed on to the Natural History Museum and the National Marine Aquarium in Plymouth."