The medieval Knepp Castle (sometimes referred to as 'Old Knepp Castle', to distinguish it from the nearby 19th-century mansion) is to the west of the village of West Grinstead, in the parish of Shipley, West Sussex, England near the River Adur and the A24 (grid reference TQ163209). The castle was probably founded by the Braose family in the 12th century. King John confiscated the castle along with the Braose lands in 1208. Knepp was used as a hunting lodge, and John visited the castle several times. He ordered its destruction in both 1215 and 1216 during the First Barons' War.

Knepp Castle continued to be used into the 14th century and hosted reigning monarchs on several occasions. The castle eventually fell out of use, and by the early 18th century was mostly destroyed. Later that century, stone from the castle was used to build a nearby road. The land around the castle is now the site of Knepp Wildland.

The name is thought to come from the Old English word cnæp, referring to the mound on which the castle stands.

Knepp was a motte castle, probably founded by William de Braose, 1st Lord of Bramber, in the Rape of Bramber. His descendant, William de Braose, 4th Lord of Bramber, was a favourite of King John and often accompanied the king early in his reign. In 1208 John confiscated William de Braose's land, including Knepp. While John's motivation is uncertain, he came to view William de Braose as a threat; in historian Sidney Painter's view the treatment of the Braose family was "the greatest mistake John made during his reign" because "it made his cruelty known to all his barons", and contributed to the discontent of the barons who later revolted against John's rule.

The first surviving record of the castle is dated to 1210. Royal records document spending at the castle in the 1210s on general repairs, building a chimney, and repairing a pond or moat. John was present at Knepp Castle on 6 April 1211, as evidenced by a charter confirmed on that date. Queen Isabella also stayed at the castle for eleven days in either 1214 or 1215.

In 1215, John was at war with the barons of England. Shortly after losing control of London, John wrote to Roland Bloett on 18 May instructing him to remove whatever he could carry from Knepp and send it to Bramber Castle, and then "totally destroy" (slight) Knepp Castle. Four days later the king again wrote to Roland, this time instructing him to welcome the Earl of Surrey, William de Warrene, at the castles of Knepp or Bramber; the historian Richard Symonds suggests this means the earlier destruction order was not followed. The sealing of Magna Carta in June 1215 brought a lull in hostilies that would prove to be temporary. Giles de Braose, Bishop of Hereford, had joined the rebellion against John and negotiated the return of the Braose family's lands in exchange for a fine in October 1215, but died before the transfer was complete and the lands remained with the king. On 13 June 1216, during renewed conflict between the king and the barons, John issue a second order to destroy the castle, instructing Roland Bloett to "cause the castle of Cnappe [Knepp], without delay, to be burnt and destroyed".

After King John died in October 1216, his son was crowned as Henry III and many barons changed sides in the war, supporting Henry against Prince Louis of France who had arrived in England with a claim to the throne and initially at the invitation of the rebelling barons. In early 1217, William Marshal, travelled through southern England dealing with Louis' supporters; Knepp Castle surrendered to Marshal during this phase of the war.

In addition to John the castle had a succession of royal visitors, including Henry III in 1218, Edward II in 1324 and Richard II in 1384. In the 13th century, Simon de Montfort seized Knepp Castle from William de Braose, and later returned it. Subsequently, it fell into decline and deteriorated.