Allier (French pronunciation: [a.lje]; Occitan: Alèir) is a department in central France named after the river Allier.

Allier is one of the original 83 departments created during the French Revolution on 4 March 1790. It was created from parts of the former provinces of Auvergne and Bourbonnais.

In 1940, the government of Marshal Philippe Pétain chose the town of Vichy as its capital. Vichy also became the department's second sub-prefecture in 1940, since the department now found itself split by the demarcation line between the occupied and (relatively, at least initially) free zones of France.

The department belongs to the region Auvergne and is surrounded by the following French departments: Cher, Nièvre, Saône-et-Loire, Loire, Puy-de-Dôme, and Creuse.

The following rivers run through Allier:

Allier is the most productive agricultural area of the Auvergne. Vichy has long been known for its water, which is exported worldwide.

Montluçon, Vichy, and Moulins are the principal cities. The rest of the department consists of smaller towns, mostly along the rivers. In general, the department is sparsely populated. The population increased until the end of the 19th century because of the growth of industry in Montluçon and Moulins and development of the thermal resources at Vichy. At that time the department had over 420,000 inhabitants. After the losses of World War I, the population stagnated, with a small increase in the 1960s. Since then, it has decreased slightly from 386,533 in 1968 to 344,721 in 1999.