Sound technician Adam Huckett proposed to the actress in Paris and she immediately said yes, according to The People.
King, who plays Carla Connor, called her family to tell them the good news as she continues her late summer break in the French capital, after recently visiting Ibiza.
A friend said: "She is over the moon. They've been together three years and Alison is sure Adam's the one for her. They're very much in love."
The couple, who met in 2007, have a daughter together - 2-year-old Daisy.
Another source added: "Adam's a brilliant dad to Daisy. They are very happy, loving parents and this engagement is the icing on the cake for Alison."
King recently admitted that Carla's dramatic storylines have left her "physically and mentally exhausted".