Alentejo (/ˌælənˈteɪʒoʊ/ AL-ən-TAY-zhoh, UK also /-ʒuː/ -⁠zhoo, Portuguese: [ɐlẽˈtɛʒu] ^ⓘ) is a geographical, historical, and cultural region of south–central and southern Portugal. In Portuguese, its name means "beyond the Tagus" (além do Tejo).

Alentejo includes the regions of Alto Alentejo and Baixo Alentejo. It corresponds to the districts of Beja, Évora, Portalegre, and Alentejo Litoral. Its main cities are Évora, Beja, Sines, Serpa, Estremoz, Elvas, and Portalegre.

It has borders with Beira Baixa in the north, with Spain (Andalucia and Extremadura) in the east, Algarve in the south, and the Atlantic Ocean, Ribatejo, and Estremadura in the west.

Alentejo is a region known for its traditional polyphonic singing groups, similar to those found in Tuscany, Corsica, and elsewhere.

In the 19th century, the comarca of the Alentejo became the Alentejo Province, divided into upper (Alto Alentejo Province) and lower (Baixo Alentejo Province) designations. The modern NUTS statistical region, Alentejo Region, was expropriated from the medieval provinces and historical territories of Estremadura Province (specifically the 1936 portions of the Ribatejo).

The term Entre-Tejo-e-Guadiana has become obsolete; it referred to roughly the same land area between the Tagus and the Guadiana rivers as part of the Kingdom of Portugal.^{[citation needed]}

Alentejo's area extends to 27,272 square kilometres (10,530 square miles) (29.6% of the country) and has a population of 537,556 (5.1% of the country). Excluding Ponte de Sor, its area is 26,432 km² (10,205 sq mi) and its population 520,834. The population density of Alentejo is 19.1 per square kilometre (49/sq mi).

Topographically, the countryside varies from the open rolling plains of the south of Alentejo to the granite hills that border Spain in the northeast. To feed the water needs of this considerable area, a number of public dams have been constructed, most notably the Alqueva Dam.