William de Ros, 6th Baron Ros

William de Ros, 6th Baron Ros (c. 1370 – 1 November 1414), was a medieval English nobleman, politician and soldier. The second son of Thomas de Ros, 4th Baron Ros, and Beatrice Stafford, William inherited his father's feudal barony and estates (with extensive lands centred on Lincolnshire) in 1394. Shortly afterwards, he married Margaret, daughter of John FitzAlan, 1st Baron Arundel. The Fitzalan family, like that of de Ros, was well-connected at the local and national level. They were implacably opposed to King Richard II, and this may have soured Richard's opinion of the young de Ros.

The late 14th century was a period of political crisis in England. In 1399, Richard II confiscated the estates of his cousin, Henry Bolingbroke, Duke of Lancaster, and exiled him. Bolingbroke invaded England several months later, and de Ros took his side almost immediately. Richard's support had deserted him; de Ros was alongside Henry when Richard surrendered his throne to the invader, who became King Henry IV. De Ros later voted in the House of Lords for the former king's imprisonment. De Ros benefited from the new Lancastrian regime, achieving far more than he had ever done under Richard. He became an important aide and counsellor to King Henry and regularly spoke for him in Parliament. He also supported Henry in his military campaigns, participating in the invasion of Scotland in 1400 and assisting in the suppression of the rebellion of Richard Scrope, Archbishop of York, five years later.

In return for his loyalty to the new regime, de Ros received extensive royal patronage. This included lands, grants, wardships, and the right to arrange the wards' marriages. De Ros performed valuable service as an advisor and ambassador (perhaps most importantly to Henry, who was often in a state of near-penury; de Ros was a wealthy man, and regularly loaned the crown large amounts of money). Important as he was in government and the regions, de Ros was unable to avoid the tumultuous regional conflicts and feuds which were rife at this time. In 1411 he was involved in a land dispute with a powerful Lincolnshire neighbour, and narrowly escaped an ambush; he sought and received redress in Parliament. Partly because of de Ros's restraint in not seeking the severe penalties available to him, he was described by a 20th-century historian as a particularly wise and forbearing figure for his time.

King Henry IV died in 1413. De Ros did not long survive him, and played only a minor role in government during the last year of his life. He may have been out of favour with the new king, Henry V. As Prince of Wales, Henry had fallen out with his father a few years before, and de Ros had supported Henry IV over his son. De Ros died in Belvoir Castle on 1 November 1414. His wife survived him by twenty-four years; his son and heir, John, was still a minor. John later fought at Agincourt in 1415 and died childless in France in 1421. The barony of de Ros was then inherited by William's second son, Thomas, who also died in military service in France, seven years after his brother.