Bas-Rhin is a département in Alsace which is a part of the Grand Est super-region of France. The name means 'Lower Rhine', referring to its lower altitude among the two French Rhine departments: it is downstream of the Haut-Rhin department. Both belong to the European Upper Rhine region. It is, with the Haut-Rhin, one of the two departments of the traditional Alsace region which until 1871, also included the area now known as the Territoire de Belfort. The more populous and densely populated of the pair, it had 1,152,662 inhabitants in 2021. The prefecture is based in Strasbourg. The INSEE and Post Code is 67.

In the administrative divisions of France, the department is one of the three levels of government under the national level, between the administrative regions and the communes. Ninety-six departments are in metropolitan France, with an additional five constituting overseas departments, which are also classified as overseas regions. Departments are further subdivided into 333 arrondissements and 2,054 cantons. These last two levels of government have no political autonomy, instead serving as the administrative basis for the local organisation of police, fire departments as well as, in certain cases, elections.