Mostazal (Spanish pronunciation: [mostaˈsal]) is a Chilean commune in Cachapoal Province, O'Higgins Region.

According to the 2002 census of the National Statistics Institute, Mostazal has 21,866 inhabitants (11,038 men and 10,828 women).

[2] As a commune, Mostazal is a third-level administrative division of Chile administered by a municipal council, headed by an alcalde who is directly elected every four years.

The council has the following members:[1] Within the electoral divisions of Chile, Mostazal is represented in the Chamber of Deputies by Eugenio Bauer (UDI) and Ricardo Rincón (PDC) as part of the 33rd electoral district, together with Graneros, Codegua, Machalí, Requínoa, Rengo, Olivar, Doñihue, Coinco, Coltauco, Quinta de Tilcoco and Malloa.

The commune is represented in the Senate by Andrés Chadwick Piñera (UDI) and Juan Pablo Letelier Morel (PS) as part of the 9th senatorial constituency (O'Higgins Region).