Allier is a department in the Auvergne-Rhône-Alpes region that borders Cher to the west, Nièvre to the north, Saône-et-Loire and Loire to th…Allier is a department in the Auvergne-Rhône-Alpes region that borders Cher to the west, Nièvre to the north, Saône-et-Loire and Loire to the east, Puy-de-Dôme to the south, and Creuse to the south-west. Named after the river Allier, it had a population of 334,872 in 2021. Moulins is the prefecture; Montluçon and Vichy are the subprefectures. Its INSEE and post code is 03.