San Marino (sän märē'nō) [key], officially Republic of San Marino, republic (2005 est. pop. 28,900), 24 sq mi (62 sq km), in the Apennines near the Adriatic Sea, SW of Rimini, N central Italy. It is the world's smallest republic and claims to be Europe's oldest existing state. The capital is San Marino (1988 est. pop. 4,140); Serravalle is the only other town.