Saarlouis is a town in Saarland, Germany, capital of the district of Saarlouis. In 2020, the town had a population of 34,409. Saarlouis is located on the river Saar. It was built as a fortress in 1680 and was named after Louis XIV of France.

The Saar is a river in northeastern France and western Germany, and a right tributary of the Moselle. It rises in the Vosges mountains on the border of Alsace and Lorraine and flows northwards into the Moselle near Trier. It has two headstreams, that both start near Mont Donon, the highest peak of the northern Vosges. After 246 kilometres (153 mi) the Saar flows into the Moselle at Konz (Rhineland-Palatinate) between Trier and the Luxembourg border. It has a catchment area of 7,431 square kilometres (2,869 sq mi).