Leiria (European Portuguese pronunciation: [lɐjˈɾi.ɐ] ^ⓘ) is a city and municipality in the Central Region of Portugal. It is the 2nd largest city in that same region, after Coimbra, with a municipality population of 128,640 (as of 2021) in an area of 565.09 square kilometres (218.18 sq mi). It is the seat of its own district and the Roman Catholic Diocese of Leiria-Fátima. The city is part of the historical province of Beira Litoral.

The region around Leiria has long been inhabited although its early history is obscure. The first evident inhabitants were the Turduli Oppidani, a Celtici tribe (akin to the Lusitanians), who established a settlement near (around 7 km) present-day Leiria. This settlement was later occupied by the Romans, who expanded it under the original Celtiberian name Collippo. The stones of the ancient Roman town were used in the Middle Ages to build much of Leiria.

The name "Leiria" in Portuguese derives from 'leira' (from the medieval Galician-Portuguese form 'laria', from proto-Celtic *ɸlār-yo-, akin to English 'floor(ing)', Old Irish 'làr' 'ground, floor', Breton 'leur' 'ground', Welsh 'llawr' 'floor') meaning an area with small farming plots. It was occupied by the Suebi in 414 until they were forced by the Romans to retreat to Galicia and later incorporated by Leovigild into the Visigoths kingdom in 585 A.D. Later the Moors occupied the area until it was re-captured by the first King of Portugal, Afonso Henriques in 1135, during the Reconquista.

South of Leiria in that period was the so-called "no-man's land", until regions further south (like Santarém and Lisbon) were permanently taken and re-populated by the Christians. In 1142 Afonso Henriques gave Leiria its first foral (compilation of feudal rights) to stimulate the colonisation of the region.

Both Afonso I of Portugal and Sancho I rebuilt the walls and the Leiria Castle to avoid new enemy incursions. Most of the population lived inside the protective city walls, but already in the 12th century part of the population lived outside the walls. The oldest church of Leiria, the Church of Saint Peter (Igreja de São Pedro), built in romanesque style in the last quarter of the 12th century, served the parish located outside the walls.

During the Middle Ages the importance of the village increased, and it was the setting of several cortes (feudal parliaments). The first of the cortes held in Leiria took place in 1245, under King Afonso II. In the early 14th century, the king restored the keep tower of the citadel of the castle, as can be seen in an inscription in the tower. He also built a royal residence in Leiria (now lost), and lived for long periods in the town, which he donated as feud to his wife, Elizabeth of Portugal (Rainha Isabel).

The king also ordered the plantation of the famous Pine Forest of Leiria (Pinhal de Leiria) near the coast. Later, the wood from this forest would be used to build the ships used in the Portuguese Navigations of the 15th and 16th centuries.

In the late 14th century, King John I built a royal palace within the walls of the castle of Leiria. This palace, with elegant gothic galleries that offered wonderful views of the town and surrounding landscape, was totally in ruins but was partially rebuilt in the 20th century.