Seia is a municipality in Guarda District in Portugal. The population in 2011 was 24,702, in an area of 435.69 km². Its urban population is about 7,000. Seia was elevated to city status on July 3, 1986. The municipality is situated on the northwestern slope of Serra da Estrela, the highest mountain range in mainland Portugal, with a top height of 1993 meters.