The Allier ( , , ; ) is a river in central France. It is a left tributary of the Loire. Its source is in the Massif Central, in the Lozère department, east of Mende. It flows generally north. It joins the Loire west of the city of Nevers. It is long, and has a drainage basin of .
The Allier flows through the following departments, and along the following towns, from source to mouth:
The main tributaries of the Allier are:
The Allier is one of the rare places in southern Europe where the freshwater grayling (Thymallus thymallus), known in French as , occurs in a natural habitat.
Grayling like to live in shoals and are sensitive to pollution. In the Allier these fish are more abundant in the stretch between Langogne and Brioude. They are economically important, being appreciated for food and fished for sport.