Casa Sommer is a building in Cascais, on the Portuguese Riviera, which houses the Cascais Municipal Historical Archive and a branch of the Cascais Municipal Library, hosting collections pertaining to the town's archaeological, architectural, historical and cultural heritage.

The exterior features smooth and curved pediments, fluted pilasters, and triglyphs.

The main façade features a rectangular portico, which frames the entrance and creates the balcony on the second floor, which is protected by a balustrade.

The work, led by the architect Paula Santos, which also included the building's old coach house, succeeded in adding considerable internal floor space.

[1] On display is the restored Foral of Cascais, which was a royal document signed by Manuel I of Portugal whose purpose was to establish a town or village Council and regulate its administration, borders and privileges.