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.

In this municipality is located the highest point in mainland Portugal, Torre, in Serra da Estrela, with 1,993 meters of altitude.

It is the largest of the lakes in the upper massif and its high hydroelectric potential led to the construction of the dam in 1911, being one of the first engineering works of this nature carried out in Portugal.

The primitive human presence on where Seia lies today dates back to pre-Roman times, when a settlement was founded by the Turduli, around the 4th century BC, under the name Senna.

When the Roman invasion of the Iberian Peninsula took place, the Lusitanians made the mountain range, then called Montes Hermínios, their headquarters, which became a strong hurdle for the invaders.

At the time of the Christian Reconquest of the Iberian Peninsula, the town was definitively conquered from the Moors by Fernando Magno, who rebuilt its fortification.

During the time of formation of the Portuguese nationality, Bermudo Peres, Teresa of León's brother-in-law, started a revolt in the castle of Seia.