Demyanyuk, Dem'yanyuk or Demjanjuk () is a patronymic surname of Ukrainian origin, meaning "child of Demyan". Notable people with the surname include:
Dmytro Dem'yanyuk (born 1983), Ukrainian high jumper John Demjanjuk (1920–2012), Ukrainian-American Nazi concentration camp guard Oleksiy Demyanyuk (1958–1999), Soviet high jumper