Robert Fitzwalter (died 9 December 1235) was one of the leaders of the baronial opposition against King John, and one of the twenty-five sureties of Magna Carta. He was feudal baron of Little Dunmow, Essex and constable of Baynard's Castle, in London, to which was annexed the hereditary office of castellan and chief knight banneret of the City of London. Part of the official aristocracy created by Henry I and Henry II, he served John in the wars in Normandy, in which he was taken prisoner by King Philip II of France and forced to pay a heavy ransom.

Fitzwalter was implicated in the baronial conspiracy of 1212. According to his own statement the king had attempted to seduce his eldest daughter, but Robert's account of his grievances varied from time to time. The truth seems to be that he was irritated by the suspicion with which John regarded the new baronage. Fitzwalter escaped a trial by fleeing to France. He was outlawed, but returned under a special amnesty after John's reconciliation with the pope.

Fitzwalter continued, however, to take the lead in the baronial agitation against the king, and upon the outbreak of hostilities in 1215 was elected "Marshal of the Army of God and Holy Church". It was due to his influence in London that his party obtained the support of the city and used it as their base of operations. The clause in Magna Carta prohibiting sentences of exile, except as the result of a lawful trial, refers more particularly to his case. He was one of the twenty-five barons appointed to enforce the promises of Magna Carta, and his aggressive attitude was one of the causes which contributed to the revival of civil war later in 1215.

He was one of the envoys who invited Prince Louis to England, and was the first of the barons to do homage when Louis entered London. Slighted by the French as a traitor to his natural lord, he served Louis with fidelity until he was captured at the Battle of Lincoln in May 1217. Released on the conclusion of peace, he joined the Fifth Crusade, but returned at an early date to make his peace with the regency. The remainder of his life was uneventful, and he died peacefully in 1235. He was the father of three children: Matilda, Robert, and Christina (who married William FitzGeoffrey de Mandeville, 3rd Earl of Essex). He is remembered as a champion of English liberty, and has also become associated with various legends, including that of Robin Hood.

Robert Fitzwalter was the son of Walter Fitz Robert of Woodham Walter and his wife Maud (or Mathilde), the daughter of Richard de Lucy of Diss (a member of the de Lucy family). Robert was a feudal baron of the fourth generation after the Norman Conquest, great-grandson of Richard fitz Gilbert (d. c. 1090). His paternal grandfather was Richard fitz Gilbert's son Robert Fitz Richard, steward of Henry I, to whom the king had granted the lordship of Dunmow and of the honour or soke of Baynard's Castle in the southwest angle of the City of London, to which the hereditary office of castellain and chief banneret of the City of London was annexed, both of which had become forfeited to the crown by William Baynard. While steward Robert may have been descended from the powerful Norman counts of Brionne, among the higher ranks of the nobility of the Norman Conquest, the house of Fitzwalter belongs properly to the administrative families, who in the latter part of the twelfth century had stepped into the place of the old feudal houses. The house of Fitzwalter's possession of the soke of Baynard's Castle, which grew into an ordinary ward, brought it into intimate relations with the Londoners. Robert Fitzwalter was himself engaged in trade, and owned wine ships which received special privileges from King John.

Nothing of Fitzwalter's birth and early life is recorded. A possible early record of him is a mention of a knight named "Robert Fitzwalter" at a tournament in Henry the Young King's retinue in 1180 at Lagny-sur-Marne. Fitzwalter was married to Gunnor or Gunnora, daughter and heiress of Robert de Valognes, some time before his father died in 1198. His eldest son, named Robert Fitzwalter, junior, was taken prisoner along with him at Lincoln, but died before him. At his death, his heir, Walter, was under age, so that the son who fought with him at Lincoln must have been dead. This Sir Walter Fitzwalter (also known as Fitz Robert) of Dunmow Castle (c. 1222–1258), married to Ida Longespée, must have been either a younger son or a grandson. After the death of Gunnor (she was alive in 1207) it is said that Fitzwalter married a second wife, Rohese, who survived him. He had also a daughter, Christina, who married William FitzGeoffrey de Mandeville, 3rd Earl of Essex.

When Baron Walter died in 1198, Fitzwalter succeeded to his estates, being already more than of full age and married to his first wife, Gunnor. His marriage to Gunnor had brought him 30½ knight's fees, and he inherited more than 66 from his father. He also acquired two knight's fees through Gunnor's uncle Geoffrey of Valognes, and about 1204 obtained livery of seisin of the lands of his own uncle, Godfrey de Luci, bishop of Winchester. Francisque Xavier Michel said that Fitzwalter came to be "one of the greatest men in England, and one of the most powerful". His lands were mainly situated in the north, so that his interests now became close to those of a faction called the "Aquilonares", whom he would later lead in the struggle against King John.

In the years following 1200, Fitzwalter is recorded as a participant in several lawsuits. In 1200 Fitzwalter was surety for half the fine incurred by his brother, Simon Fitzwalter, for marrying without royal licence. In 1201 he made an agreement in the curia regis with St Albans Abbey with respect to the wood of Northaw. Another suit sprang from his claim to the custody of the Hertford Castle as of ancient right. He withdrew this suit for a time, and in August 1202 King John made Fitzwalter warden of Hertford Castle by royal letters patent, releasing him from his family's debts to Jewish moneylenders as well.