John du Plessis or Plessetis, Earl of Warwick (died 26 February 1263) was an Anglo-Norman nobleman in the service of King Henry III of England.
He was at Bordeaux in August 1254, but, having obtained letters of safe-conduct from Louis IX, started home through Poitou early in September, in company with Gilbert de Segrave and William Mauduit.
The party was treacherously seized by the citizens of Pons in Poitou; Segrave died in captivity, and John du Plessis was not released until the following year.
When Henry removed the baronial sheriffs in July 1261, du Plessis was given charge of Leicestershire, and on 10 August was also made warden of Devizes Castle, a post which he held till 15 June 1262.
John du Plessis's first wife was Christiana (died before 1242), daughter and heiress of Hugh de Sanford of Hook Norton.
The latter was a person of some note in the county, and was no doubt the warden of Northumberland in 1258, though Dugdale and others have erroneously assigned this office to the Earl of Warwick.
The descendants of John du Plessis generally adopted the Latinized form but dropped the preposition, and over the centuries the surname was corrupted into various spellings, including Pleisted, Plaisted, Playsteed, Pleastid and Plestead.