The faint scratchings of a speared reindeer are believed to have been carved by an ancient hunter-gatherer and are "very, very exciting," George Nash of Bristol University said.
Nash made the discovery while exploring the caves on Wales' Gower peninsula in September 2010.
"For 20-odd years I have been taking students to this cave and talking about what was going on there," he said.
"They went back to their cars and the bus and I decided to have a little snoop around in the cave as I've never had the chance to do it before.
"Within a couple of minutes I was scrubbing at the back of a very strange and awkward recess and there a very faint image bounced in front of me -- I couldn't believe my eyes," he said.
Although the reindeer drawing is similar to many found in northern Europe around 4,000-5,000 years later, the discovery of flint tools in the cave decades ago could indicate the carving's true date, he said.
"In the 1950s, Cambridge University undertook an excavation there and found 300-400 pieces of flint and dated it to between 12,000-14,000 B.C.," Nash said.
The location of the find, now being officially dated and verified by experts at the National Museum of Wales and Cadw, will be revealed to the public in the future, the BBC reported.