The Fort of Nossa Senhora da Guia, also referred to as Forte da Guia Fortress of Greta or Castle Greta, is located on the tip of Greta, parish of Angústias in the city and county of Horta on Faial in the Azores.

Erected on the escarpment of Monte da Guia, in a dominant position over the Bay of Horta was part of the defensive system of the island against pirates and privateers attacks, once common in this Atlantic region. It is currently inserted in the area of protected landscape of Monte da Guia.