Santa Cruz (Cape Verde): There are many bonds based on culture and friendship that link the Portuguese people and the people of Africa who have Portuguese as their official language. Sines and Santa Cruz were paired in 1993; it was a way of celebrating these bonds as well as the fact that there is a large community of people from Cape Verde living in Sines. Having a twin means establishing cultural, social, educational, tourist and economic cooperation. However, the most important goals foreseeing cooperation include technical cooperation, where students come and go on exchange trips and where local development is financed.

Évora-Vidigueira-Nisa (Portugal): Sines, Évora, Vidigueira and Nisa were the four cities where Vasco da Gama lived while he was in Portugal. The 1995 agreement to become twin cities aims at furthering cultural cooperation among these cities.