Spa (French pronunciation: [spa] ^ⓘ; Walloon: Spå) is a municipality and city of Wallonia in the province of Liège, Belgium, whose name became an eponym for mineral baths with supposed curative properties. It is in a valley in the Ardennes mountains 35 km (22 mi) south-east of Liège and 45 km (28 mi) south-west of Aachen. In 2006, Spa had a population of 10,543 and an area of 39.85 km² (15.39 sq mi), giving a population density of 265/km² (690/sq mi).

Spa is one of Belgium's most popular tourist destinations, being renowned for its natural mineral springs, and production of "Spa" mineral water, which is exported worldwide. The motor-racing Circuit de Spa-Francorchamps, just south of the nearby village of Francorchamps, also hosts the annual Formula One Belgian Grand Prix and various endurance races such as the 24 Hours of Spa. The world's first beauty pageant, the Concours de Beauté, was held in Spa on 19 September 1888. The town also hosted the Tour de France on 5 July 2010, when stage 2 of the race ended there.

In 2021, Spa became part of the transnational UNESCO World Heritage Site the Great Spa Towns of Europe, for its famous mineral springs and architectural testimony to the rise of European bathing culture in the 18th and 19th centuries.

Among the various hypotheses put forward as to the etymological origin of the name Spa is that of "gushing spring", from the Latin sparsa meaning "scattered" and "gushing", past participle of spargere ("scatter", "sprinkle" or "moisten"). Another connects the word to the meaning of "free space", from the Walloon spâ and from the Latin spatia, plural of spatium.

The place has been known since Roman times, when the location was called Aquae Spadanae. For this reason, the city's name is sometimes presented as the acronym of various Latin phrases, such as salus per aquam or sanitas per aquam, meaning "health through water"; it is in reality a backronym, an a posteriori fabrication of a fictitious acronym.

The term Spa has since become eponymous with any place having a natural water source that is believed to possess special health-giving properties, known as a spa.

As the site of cold springs with alleged healing properties, Spa has been frequented as a "water-taking" place since classical antiquity. Pliny the Elder (died 79 CE) noted, "There is a famous spring in Tungri, a state of Gaul, whose water, sparkling with bubbles, has a ferruginous taste that is only noticeable when the drink is finished. This water purges the body, cures third-grade fevers and dispels calculous affections. The same water, put on the fire, becomes cloudy and eventually turns red." (C lib. XXXI VIII)

The spa town grew in the Middle Ages, in the oldest iron and steel centre of Liège Province. The ban Spa was created around 1335 and included two urban concentrations: vilhe of Creppe and vilhe Spas, 2 km (1.2 mi) away. Prior to the exploitation of mineral water, the steel industry developed communication lines, which made it possible to develop the spa town.^{[citation needed]}