The Spanish language, also called the Castilian language, is a Western Romance language that originated in the Castile region of Spain and today has hundreds of millions of native speakers in Latin America and Spain. It is usually considered the world's second-most spoken native language, after Mandarin Chinese.

It is estimated that more than 437 million people speak Spanish as a native language, which qualifies it as second on the lists of languages by number of native speakers. Instituto Cervantes claims that there are an estimated 477 million Spanish speakers with native competence and 572 million Spanish speakers as a first or second language—including speakers with limited competence—and more than 21 million students of Spanish as a foreign language.

Spanish is the official or national language in Spain, Equatorial Guinea, and 19 countries in the Americas. Speakers in the Americas total some 418 million. In the European Union, Spanish is the mother tongue of 8% of the population, with an additional 7% speaking it as a second language. Spanish is the most popular second language learned in the United States. In 2011 it was estimated by the American Community Survey that of the 55 million Hispanic United States residents who are five years of age and over, 38 million speak Spanish at home.

Spanish is one of the six official languages of the United Nations. It is also used as an official language by the European Union, the Organization of American States, the Union of South American Nations, the Community of Latin American and Caribbean States, the African Union and by many other international organizations.

Spanish is a part of the Ibero-Romance group of languages, which evolved from several dialects of Vulgar Latin in Iberia after the collapse of the Western Roman Empire in the 5th century. The oldest Latin texts with traces of Spanish come from mid-northern Iberia in the 9th century, and the first systematic written use of the language happened in Toledo, then capital of the Kingdom of Castile, in the 13th century. Beginning in the early 16th century, Spanish was taken to the colonies of the Spanish Empire, most notably to the Americas, as well as territories in Africa, Oceania and the Philippines.

Around 75% of modern Spanish vocabulary is derived from Latin. Ancient Greek has also contributed substantially to Spanish vocabulary, especially through Latin, where it had a great impact.

Spanish vocabulary has been in contact with Arabic from an early date, having developed during the Al-Andalus era in the Iberian Peninsula. With around 8% of its vocabulary being Arabic in origin, this language is the second most important influence after Latin. It has also been influenced by Basque, Celtiberian, Visigothic, and by neighboring Ibero-Romance languages. Additionally, it has absorbed vocabulary from other languages, particularly the Romance languages French, Italian, Occitan, and Sardinian, as well as from Nahuatl, Quechua, and other indigenous languages of the Americas.