Taff's Well thermal spring is located in Taff's Well (Welsh: Ffynnon Taf) north of Cardiff, south Wales (National Grid Reference ST 11925 83639) and an elevation of about 30 m. It is the only thermal spring in Wales.

The spring emerges on the eastern bank of the River Taff and has been contained within a brick well structure and building.

There is one visible overflow pipe which emerges several meters to the west of the spring, on the bank of the River Taff.

[7] The spring reached its height of popularity during the mid to late 1800s, with visitors arriving in the village in large numbers hoping to cure their ailments.

Hall concluded that the waters relieved and occasionally cured chronic disorders, citing a young man who came as a cripple and left after a fortnights bathing able "to run about the green meadow and enjoy life.

”But how” we inquired, “how is it that with such a number of holy wells in your own country you leave them and come to St. Taff to be cured?” “Because I’m for justice to ould Ireland.

[9] The growing popularity of Taff's Well spring as a tourist destination was again reported in 1877 by the chemist J. W. Thomas [10] who said ‘the well waters have long since obtained some celebrity, especially the well water as a curative agent for rheumatism.’ Thomas sensibly concludes ‘we do not feel encouraged by this story (which, by the way, we rather fancy we have heard before) to insist very much upon the curative properties of the Water of Taff's Well.’ During the 19th century a weir was constructed across the River Taff.

Rhondda Cynon Taff Council had plans to reinstate the spring and to utilise it as a tourist attraction but these have not been fully realised.

A lady robed in grey frequently visited this well, and many people testified to having seen her in the twilight wandering along the banks of the river near the spring, or going on to the ferry under the Garth Mountain.

The last was to the effect that about seventy or eighty years ago the woman in grey beckoned a man who had just been getting some of the water.

He began to think it a long time before she bade him cease his grip, when a "stabbing pain" caught him in his side and with a sharp cry he loosed his hold.

A survey of thermophillic cyanobacteria identified abundant growths of Phormidium ambiguum GOM which formed conophyton like pinnacles up to 40 cm high in the still water.

Filamentous algae Melosira variants and Spirogyra have both been identified [1] and are very abundant in the flooded pool area outside of the well building.