The 26-year-old England international has signed a two-year deal with the French side.
She will link up with former Manchester City teammate Lucy Bronze, who has already made a name for herself as one of the irreplaceable first team players.
Bronze joined Lyon in 2017, and they went on to win the third successive Champions League title the following May.
July 23, 2018
Christiansen told the Lyon website: "I cannot wait to start with my new team. It is a great club, champions of France and Europe."
"I am very proud to join this club. I have come to OL to win titles, progress and have fun."
Lyon will be facing Manchester City in Miami on Thursday, in the inaugaral Women's International Champions Cup pre-season tournament.