Belles boss Emma Coates said: “Losing Courtney is disappointing but her goal scoring record was bound to generate interest. She’s a top player and we wish her all the best for the coming season.”
Everton paid an undisclosed fee for Sweetman-Kirk.
Sweetman-Kirk, 26, said: “I want to say a big thank you to the club, all the staff, players and the fans for their support whilst I have been at the Belles.
“I have some of my best footballing memories at the club but I feel this is the right move for me to fulfil my WSL 1 and international ambitions.”
Long serving midfielder Kasia Lipka has also left the Belles to join FAWSL1 side Sunderland.
Belles, who begin their FAWSL2 campaign at London Bees on September 24, have signed former Notts County and England defender Sophie Bradley-Auckland, while England Under 19 international midfielder Chloe Peplow has joined on loan from Birmingham City.