John Llewellyn may refer to:

• John Llewellyn (fencer) (born 1957), British fencer who appeared at three Olympic Games
• John Llewellyn (racing driver), British former auto racing driver
• Sir John Llewellyn (academic), academic administrator in New Zealand and Great Britain
• John Anthony Llewellyn (1933–2013), British-born American scientist and NASA astronaut
• Sir John Dillwyn-Llewellyn, 1st Baronet (1836–1927), Welsh Conservative Member of Parliament
• John Seys-Llewellyn (1912–2003), Welsh barrister and judge
• John Llewellin, 1st Baron Llewellin (1893–1957), British army officer and politician

• John Llewelyn (born 1928), Welsh-born British philosopher
• John Dillwyn Llewelyn (1810–1882), Welsh botanist and pioneer photographer