The Portuguese nobility was a social class enshrined in the laws of the Kingdom of Portugal with specific privileges, prerogatives, obligations and regulations. The nobility ranked immediately after royalty and was itself subdivided into a number of subcategories which included the titled nobility and nobility of blood at the top and civic nobility at the bottom, encompassing a small, but considerable proportion of Portugal's citizenry.

The nobility was an open, regulated social class. Accession to it was dependent on a family's or, more rarely, an individual's merit and proven loyalty to the Crown in most cases over generations. Formal access was granted by the monarch through letters of ennoblement and a family's status within the noble class was determined by continued and significant services to Crown and country. Living outside the laws of the nobility immediately revoked an individual's status and that of his descendants.

Unlike many other European countries, power in Portugal was effectively centralised in the Crown, despite attempts to the contrary by the nobility, most notably during the reign of King João II, as was the capacity to confer nobility and other awards as well as to refuse them.

During the Portuguese monarchy, as well as enjoying the most privileged status and access to Court, members of the nobility, particularly the titled nobility, including major hierarchs of the Catholic Church, held the most important offices of State – administrative, judicial, political and military. With the needs of an ever larger global empire and the rise of mercantilism, and growth in importance of the mercantile class, privileges were increasingly widened, eroding the relative power held particularly by the titled nobility, a situation which was accelerated significantly during the reign of King José I, as a result of the policies of his prime minister, the Marquis of Pombal, himself recently elevated to the highest echelons of the nobility.

With the Portuguese Constitution of 1822 and the introduction of a constitutional monarchy, all noble privileges were extinguished, and the influence of the traditional nobility declined significantly. Notwithstanding, nobility – hereditary or otherwise – continued to be recognised in law as a status with certain prerogatives, albeit merely honorific ones, until the establishment of the Portuguese Republic in 1910.

Descendants of Portugal's hereditary nobles have continued to bear their families' titles and coats of arms according to the standards and regulations established before the Republic, and currently sustained by the Institute of Portuguese Nobility (Instituto da Nobreza Portuguesa), whose honorary president is D. Duarte Pio, Duke of Braganza, head of the House of Braganza and presumptive heir to the Portuguese throne.

The Portuguese nobility can be traced back to the reign of Alfonso VI of Leon, whose reign saw the sons of Leonese nobility established as gentry in the north of Portugal, between the Minho River and the Douro River. This was the region of the sun and the most powerful men of the kingdom. They united nobility of birth to the authority and prestige of public office.

They were followed in the hierarchy, in descending order, by infancies, cavaleiros (knights) and escudeiros (squires). A title of Spanish origin, filho de alguém, applied to senior functionaries and gave rise to the word fidalgo, who, in the 14th century, became widespread and went on to name all of noble lineage, thereby designating the highest class of the nobility, without distinction of rank.