Castro Marim (Portuguese pronunciation: [ˈkaʃtɾu mɐˈɾĩ]) is a town and a municipality in the southern region of Algarve, in Portugal. The municipality is composed of 4 parishes.

The present Mayor is José Fernandes Estevens, elected by the Social Democratic Party.

The municipal holiday is June 24.

In the Roman era, Castro Marim was known as Aesuris.

Every year in the end of August there is a Medieval Fair/Festival that reunites many people from across the world to perform, like medieval musicians, archers, swordsmen, dancers, troups, etc. There are sellers too: blacksmiths, textile crafters (weaving), herbs sellers, etc.