Jonassen is a surname. Notable people with the surname include:
Emil Jonassen (born 1993), Norwegian football playerFartein Døvle Jonassen (born 1971), Norwegian novelist and translatorHagbard Jonassen (1903–1977), Danish botanist, quaternary geologist, war resister and nuclear disarmament proponentKenneth Jonassen (born 1974), male badminton player from DenmarkVitus Jonassen Bering or Vitus Bering (1681–1741), Danish-born navigator in the service of the Russian Navy