The hand-painted floral artwork at the National Trust-run Tudor Merchant House in Tenby, is thought to date between the late 18th and early 19th Century.
But the wall is at risk of crumbling so conservationists are working to stabilise it.
They hope to uncover more artwork and to discover the identity of the artist.
The three-colour pattern was painted freehand on to the north wall of the 15th Century house's entrance hall.
The first phase of the work will see the wall strengthened to prevent it collapsing.
After that is complete, conservationists hope to peel back the plaster on the hall's remaining walls to see if the pattern continues.
Pigment analysis of the paint has indicated high-quality materials were used to create the design and National Trust said the artwork was likely painted to promote the wealth of the owners of the time.
But the artist behind the work remains a mystery and the charity hopes to identify them.
It is appealing to owners of other similar townhouses in Tenby to check if their properties have similar paintings so samples can be compared in the hope of tracing the person behind the paintbrush.
The house remains open while the work is taking place so visitors can see the team at work and learn more about the paintings.
Manager Angela Jones said the project was a "fantastic opportunity" for visitors to see conservation in action.
"By coming to see the past, you're helping us safeguard the future of the Tudor Merchant's House," she added.
Work has begun to save centuries-old wall paintings at one of Wales' oldest examples of a traditional Pembrokeshire town house.