John Deviock or Devyok (born c. 1420) was a Cornish gentleman and pirate from Ethy in the parish of St Winnow in Cornwall. In 1473 he was issued a Commission of arrayfor the lieges of Cornwall to capture St Michael's Mount, which had been taken by the John de Vere, 13th Earl of Oxford and William Beaumont, 2nd Viscount Beaumont during a siege of twenty-three weeks against 6,000 of Edward IV's troops.