Spring waters have been part of the history of this town since Roman times, when it was known as Aquis Celenis. In the old days, the Celeni and Roman people thought that the medicinal properties of these waters were associated with a god, in this case with Endovius, which means "the one who provides heat".
Two votive altars were found in the area of the historical centre of Caldas de Reis. The first one was discovered in 1798, during the construction of the spa-hotel Balneario Davila, and it was used as a manhole cover, since nobody was aware of its importance. It was rediscovered and in 1909 disappeared. The second votive altar was found in 2009, while carrying out public works on the wall of a house more than 130 years old.
Nowadays, there is a public fountain with two spouts, built by the town council in 1881, pouring water with a temperature of around 50 ºC. Also downtown, there are two spas, one on each bank of the Umia River: the Balneario Acuña, an example of the twentieth-century Galician architecture, and the Balneario Davila, boasting an amazing and unique bamboo reed bed.
Regarding the name of the town, Caldas de Reis, it has its origins in the 12th century, when Urraca, queen of León, had a defensive tower there where she used to spend long periods with her son Alfonso VII, king of Galicia, Leon and Castile. Thus, it received the name of Caldas de Rex and it became a noble town.