Charles III (Spanish: Carlos Sebastián de Borbón y Farnesio; 20 January 1716 – 14 December 1788) was King of Spain from 1759 until his death in 1788. He was also Duke of Parma and Piacenza as Charles I (1731–1735), King of Naples as Charles VII and King of Sicily as Charles III (or V) (1735–1759). He was the fourth son of Philip V of Spain and the eldest son of Philip's second wife, Elisabeth Farnese. During his reign, Charles was a proponent of enlightened absolutism and regalism in Europe.

In 1731, the 15-year-old Charles became Duke of Parma and Piacenza following the death of his childless grand-uncle Antonio Farnese. In 1734, at the age of 18, he led Spanish troops in a bold and almost entirely bloodless march down Italy to seize the Kingdom of Naples and Kingdom of Sicily and enforce the Spanish claim to their thrones. In 1738, he married the Princess Maria Amalia of Saxony, daughter of Augustus III of Poland, who was an educated, cultured woman. The couple had 13 children, eight of whom reached adulthood. They resided in Naples for 19 years. Charles gained valuable experience in his 25-year rule in Italy, so that he was well prepared as the monarch of the Spanish Empire. His policies in Italy prefigured ones he would put in place in his 30-year rule of Spain.

Charles succeeded to the Spanish throne in 1759 upon the death of his childless half-brother Ferdinand VI. As king of Spain, Charles III made far-reaching reforms to increase the flow of funds to the crown and defend against foreign incursions on the empire. He facilitated trade and commerce, modernized agriculture and land tenure, and promoted science and university research. He implemented regalist policies to increase the power of the state regarding the church. During his reign, he expelled the Jesuits from the Spanish Empire and fostered the Enlightenment in Spain. Charles launched enquiries into the Iberian Peninsula's Muslim past, even after succeeding to the Spanish throne. He strengthened the Spanish Army and the Spanish Navy. Although he did not achieve complete control over Spain's finances, and was sometimes obliged to borrow to meet expenses, most of his reforms proved successful in providing increased revenue to the crown and expanding state power, leaving a lasting legacy.

In the Spanish Empire his regime enacted a series of sweeping reforms with the aim of bringing the overseas territories under firmer control by the central government, reversing the trend toward local autonomy, and gaining more control over the Church. Reforms including the establishment of two new viceroyalties, realignment of administration into intendancies, creating a standing military, establishing new monopolies, revitalizing silver mining, excluding American-born Spaniards (criollos) from high civil and ecclesiastical offices, and eliminating many privileges (fueros) of clergy.

Historian Stanley Payne writes that Charles "was probably the most successful European ruler of his generation. He had provided firm, consistent, intelligent leadership. He had chosen capable ministers ... [his] personal life had won the respect of the people." John Lynch's assessment is that in Bourbon Spain "Spaniards had to wait half a century before their government was rescued by Charles III."

In 1713, the Treaty of Utrecht concluded the War of the Spanish Succession (1701–14) and reduced the political and military power of Spain, which the House of Bourbon had ruled since 1700. Under the terms of the treaty, the Spanish Empire retained its American territories and the Philippines, but ceded the Spanish Netherlands, the kingdoms of Naples and Sardinia, the Duchy of Milan, and the State of Presidi to Habsburg Austria. The House of Savoy gained the Kingdom of Sicily, and the Kingdom of Great Britain gained the island of Menorca and the fortress at Gibraltar.

In 1700, Charles's father, originally a French Bourbon prince, Philip of Anjou, became King of Spain as Philip V. For the remainder of his reign (1700–46), he continually attempted to regain the ceded territories in Europe. In 1714, after the death of the king's first wife, the Princess Maria Luisa Gabriella of Savoy, the Piacenza Cardinal Giulio Alberoni successfully arranged the swift marriage between Philip and the ambitious Elisabeth Farnese, niece and stepdaughter of Francesco Farnese, Duke of Parma. Elisabeth and Philip married on 24 December 1714; she quickly proved a domineering consort and influenced King Philip to make Cardinal Giulio Alberoni the prime minister of Spain in 1715.

On 20 January 1716, Elisabeth gave birth to the Infante Charles of Spain at the Royal Alcázar of Madrid. He was fourth in line to the Spanish throne, after three elder half-brothers: the Infante Luis, Prince of Asturias (who ruled briefly as Louis I of Spain before dying in 1724); the Infante Felipe (who died in 1719); and Ferdinand (the future Ferdinand VI). Because the Duke Francesco of Parma and his heir were childless, Elisabeth sought the Duchy of Parma and Piacenza for Charles, since he was unlikely to be king of Spain. She also sought for him the Grand Duchy of Tuscany, because Gian Gastone de' Medici, Grand Duke of Tuscany (1671–1737) was also childless. He was a distant cousin of hers, related via her great-grandmother Margherita de' Medici, giving Charles a claim to the title through that lineage.