Mortensen or Mortenson (in US) is a surname of Danish and Norwegian origin, meaning son of Morten. The Swedish variant is Mårtensson. Mortensen is currently the 20th most common surname in Denmark.

It may refer to the following people:

  • Arne Mortensen (1900–1942), Norwegian rower
  • Beverly Mortensen, American musician and composer
  • Carlos Mortensen (born 1972), professional poker from Ecuador
  • Christian Mortensen (1882–1998), Danish-American supercentenarian
  • Chris Mortensen (born 1951), American journalist
  • Dale T. Mortensen (born 1939), American economist
  • Greg Mortenson (born 1957), American humanitarian
  • Hans Christian Cornelius Mortensen (1856–1921), Danish teacher and ornithologist
  • Jess Mortensen (1907–1962), American NCAA champion track athlete and coach
  • Malene Mortensen (born 1982), Danish singer
  • Ole Mortensen (born 1958), Danish cricketer
  • Scott Mortensen, American politician
  • Stan Mortensen (1921–1991), English professional footballer
  • Viggo Mortensen (born 1958), Danish-American actor, poet, musician, photographer and painter
  • Marilyn Monroe (1926–1962), born Norma Jeane Mortenson