Oeiras (locally [ɔˈɐjɾɐʃ] ⓘ) is a town and municipality in the western part of Lisbon metropolitan area, located within the Portuguese Riviera, in continental Portugal.

The mild climate, access to water, quality of its soils and geographically advantageous location at the mouth of the Tagus River attracted early settlement to this region.

These industries were supported and guarded by the construction of several fortifications along a maritime defensive line that ringed the southern coast to Lisbon and that controlled navigation in the Tagus estuary from the 16th to 18th centuries.

In 1770, the first agricultural and industrial fair was established in Oeiras, representing a unique national event that also contributed to the creation of fishing shelters and a new customs house and factory, among other projects.

In the late 17th and early 18th centuries, several estates and palaces began to be constructed in order to explore agricultural production, principally cereals and vineyards, which supported the growing markets of Lisbon.

By the beginning of the 20th century, many of the beaches in Oeiras were already occupied by the higher social classes, who travelled to the municipality for medical reasons (believing the air and beach-waters were therapeutic).

The construction of National Roadway 6 (EN6) would later link Lisboa to Cascais, permitting new travellers to experience the area, resulting in an influx of new residents that expanded the urban centres, giving rise to beach "chalets" and summer cottages.

By the end of the 1980s, however, Oeiras began to constitute its own economic pole in the Lisbon metropolitan area, concentrating its development on tertiary activities linked to science, investigation, information technology and communications.

Oeiras has a Mediterranean climate with warm to hot, dry summers and mild, wet winters with less precipitation and slightly smoother variations in temperature than the nearby city of Lisbon.

The industrialized and heavily commercial municipality is occupied by several facilities, which include the headquarters of SIC and TVI (two major Portuguese television networks), as well as main operational offices, research units, subsidiaries and branches of many large multinational companies, including Google,[6] Nestlé, McDonald's, BP, General Electric, Cisco, Synopsys, P&G, Hewlett-Packard, Dell, Epson, Samsung, LG, Unit4, Philips, Intel, Colgate-Palmolive, Johnson & Johnson, L'Oréal, Oracle, Atlas Copco, ABB, Daikin, Nike, Nissan, PepsiCo, Pfizer, Grundfos, Merck & Co, NetJets Europe, Synopsys, Miele, Unisys, BMW, Toshiba, Canon, GSK, Volvo Cars, and Stellantis.

These include the principal naval service command of the North Atlantic Treaty Organization's (NATO) Allied Command Operations (ACO); IGC - Instituto Gulbenkian de Ciência, and a branch of the Instituto Nacional dos Recursos Biológicos, responsible for agrarian research, the Estação Agronómica Nacional (National Agronomy Station), which are all located within the municipality of Oeiras.

Oeiras offers a range of points of interest, ranging from monuments of unique and historical value, such as the Palace of Marquês de Pombal and Palace of the Royal Estate in Caxias (that dates from the 18th century and surrounded by luxurious gardens) to sites with industrial history, like the Fábrica da Pólvora de Barcarena ('Barcarena Gunpowder Factory'), a reclaimed arms factory used for leisure and cultural purposes.

The final matches of the Portuguese men's and women's national football cups traditionally take place in the Estádio Nacional in the Jamor complex.