Cascais (European Portuguese pronunciation: [kɐʃˈkajʃ] ^ⓘ) is a town and municipality in the Lisbon District of Portugal, located on the Estoril Coast. The municipality has a total of 214,158 inhabitants in an area of 97.40 km². Cascais is an important tourist destination. Its marina hosts events such as the America's Cup and the town of Estoril, part of the Cascais municipality, hosts conferences such as the Horasis Global Meeting.

Since the 1870s, Cascais has been a popular seaside resort after King Luís I of Portugal and the Portuguese royal family made the seaside town their residence every September, thus also attracting members of the Portuguese nobility, who established a summer community there. Cascais is known for the many members of royalty who have lived there, including King Edward VIII of the United Kingdom, when he was the Duke of Windsor, King Juan Carlos I of Spain, and King Umberto II of Italy. Former Cuban president Fulgencio Batista was also once a resident of the municipality. The Casino Estoril inspired Ian Fleming's first James Bond novel, Casino Royale.

The municipality is one of the wealthiest in both Portugal and the Iberian Peninsula. It has one of the most expensive real estate markets and one of the highest costs of living in the country, and is consistently ranked highly for its quality of life.

Human settlement of the territory today known as Cascais dates to the late Paleolithic, as indicated by remnants encountered in the north of Talaíde, in Alto do Cabecinho (Tires) and south of Moinhos do Cabreiro. It was during the Neolithic that permanent settlements were established in the region, their inhabitants utilizing the natural grottoes (such as the Caves of Poço Velho in Cascais) and artificial shelters (like those in Alapraia or São Pedro) to deposit their dead. The bodies were buried along with offerings, a practice that continued to the Chalcolithic.

Roman interventions in the area occurred with the settlement of the villae of Freiria (today São Domingos de Rana) and Casais Velhos (Charneca), evidence for which includes a group of ten tanks discovered along the Rua Marques Leal Pancada in Cascais, which was the location of a salting factory for fish. Roman dominion over the territory also influenced place names in the region, as was the case with the word "Caparide" (from the Latin capparis, meaning "caper"), as well as several inscriptions associated with funerary graves.

The Visigoths also left their mark especially on the Visigothic Cemetery of Alcoitão, as well as in the late-Roman and medieval necropolis of Talaíde.

Similarly, Muslim settlers in the region left their mark on local place names, including "Alcoitão" and "Alcabideche", where the romantic poet Ibn Muqana al-Qabdaqi, who wrote of the region's agriculture and windmills, was born at the beginning of the 11th century. The discovery of several corpses in 1987 at Arneiro, in Carcavelos, led to the identification of fifteen burials that, due to their characteristics, made it possible to verify that the individuals buried there were of Berber origin.

The development of Cascais began in earnest in the 12th century, when it was administratively subordinate to the town of Sintra, located to the north. In its humble beginnings, Cascais depended on the products of the sea and land, but by the 13th century its fish production was also supplying the nearby city of Lisbon. The toponym "Cascais" appears to derive from this period, a plural derivation of cascal (monte de cascas) which signified a "mountain of shells", referring to the abundant volume of marine mollusks harvested from the coastal waters. During the 14th century, the population spread outside the walls of its fortress castle.