Lionel of Antwerp, Duke of Clarence, (Norman: Leonell Duc de Clarence; 29 November 1338 – 17 October 1368), was an English prince, Earl of Ulster jure uxoris from 1347, Duke of Clarence from 1362, Guardian of England in 1345–46, Lord Lieutenant of Ireland in 1361–66, Knight of the Garter from 1361, third son (second surviving) of King Edward III of England and Philippa of Hainault. He was named after his birthplace, at Antwerp in the Duchy of Brabant.

In 1355–60, Lionel took part in the Hundred Years' War with France and the Second War of Scottish Independence. After the Treaty of Brétigny, much of the Prince's later career was linked to Ireland. Through his first marriage to Elizabeth de Burgh, 4th Countess of Ulster, he inherited large holdings in Northern, Western and South-Western Ireland, as well as the title of Earl of Ulster. In 1361, Edward III appointed his son the royal lieutenant (viceroy) of Ireland, and in 1362 he created the title of Duke of Clarence for him, making Lionel the first among the Irish peers. The prince remained viceroy until 1366 (with two short breaks in 1364 and 1365, when he went to England). During this time, Lionel led several military campaigns on the island and adopted the Statutes of Kilkenny in February 1366, which became his most important legacy for the Irish government.

Lionel's first wife died in 1363. In 1367, Edward III arranged a new marriage for his son with Violante Visconti, whose father was the ruler of Milan. A lavish marriage ceremony took place in May or June 1368, but Lionel died soon after, possibly poisoned by his father-in-law. He left one daughter, Philippa, from his first marriage. In the 15th century, the House of York justified their preemptive right to the English throne through their descent from Lionel and Philippa.

Lionel patrilineally descended from the English royal House of Plantagenet. His father was Edward III, King of England since 1327 and his mother was Philippa of Hainault, who came from the French Avesnes family. His paternal grandparents were King Edward II of England and Isabella of France, and his maternal grandparents were William I, Count of Hainaut, Holland and Zeeland and Joan of Valois. Edward III and Philippa had 12 children in the course of their marriage. Lionel was the fifth born child and the third but second surviving son (one of his older brothers, William of Hatfield, died in infancy in 1337). Of the other brothers, the eldest, Edward the Black Prince, was the heir to Edward III. Lionel also had four younger brothers (three of them survived infancy: John of Gaunt, Duke of Lancaster, Edmund of Langley, 1st Duke of York, and Thomas of Woodstock, Duke of Gloucester), as well as five sisters, of whom only the eldest, Isabella, Countess of Bedford, survive to adulthood.

Lionel was born on 29 November 1338 at St. Michael's Abbey in Antwerp, while his parents were living in the Low Countries. Gilles de Monte, a doctor from Hainault, took care of the newborn prince for 13 weeks, for which he received a reward of 15 £. Due to the place of birth, Lionel received the nickname "of Antwerp".

There are several versions explaining the unusual name that Edward III gave to his son. According to one of them, it is a reference to the Lion of Brabant —the heraldic symbol of John III, Duke of Brabant, one of the allies of the English king at that time. However, according to modern researchers, the choice of name is more likely due to the fact that Edward III, who adored the legends of King Arthur, at that time identified himself with one of the Knights of the Round Table —Sir Lionel, often participating in jousting tournaments with his coat of arms (for example, at a tournament in Dunstable in 1334). In addition, the name had an obvious association with the heraldic lions (more precisely, leopards) depicted on the English coat of arms. According to another version, Edward III decided to revive the "Welsh name Llywelyn".

Lionel's godfather was William Montagu, 1st Earl of Salisbury. Previously, the king had granted the earl the privilege of wearing the royal arms bearing an eagle; in 1339 Salisbury's godson received the same privilege.

Already at the age of 3, Lionel was included in the dynastic policy of his father, who sought to ensure the inheritance of his sons. Back in 1333, William de Burgh, 3rd Earl of Ulster and 6th Baron Connaught, the head of one of the largest Anglo-Norman families in Ireland, died. From his marriage to Maud of Lancaster, he left an only daughter, Elizabeth, who inherited the de Burghs' large estates in Ireland. On 9 September 1342, Lionel and Elizabeth de Burgh were married in the Tower of London. The composition of the possessions inherited by Lionel as a dowry included large holdings in Connaught, Munster, and Ulster, located in Northern, Western and Southwestern Ireland.