The former London Bees forward is Head Coach Carla Ward's latest capture.
A right winger, Toussaint suffered an ACL injury in April 2019 but is now fully fit and raring to go again.
She was a regular for the Bees, scoring eight times in the 2017/18 and 2018/19 campaigns.
“I’m so happy to be back playing after my injury and enjoying football again," said Toussaint, who has been training with Blues during pre-season.
"I’m really excited to play for Carla Ward, after knowing her for a long time and playing against her as well. I can’t wait for the new challenge this season and playing in the top league."
Toussaint was previously with Aston Villa and Oxford United when both clubs were in WSL2.