She was recently playing in Iceland for IBV Vestmannaeyjar, where she earned player-of-the-year honours.
Kelly, who turns 23 later this month, is an attacking midfielder/forward who can also play in the wide areas.
She came through at Sunderland Ladies and helped them to the inaugural WSL 2 title in 2014.
A former England Under-17 and Under-19 international, she has also been in camp with England Under-21s.
Manager Marta Tejedor commented: "We're very happy to have signed a player of Emma's calibre.
"In Iceland she was the outstanding player and received some awards.
"She can play, either nine, 10 or both wings, which is one of the qualities we expect from our players - the ability to adapt to different positions.
"She can give us more life in the forward areas, in the finishing zone if you like.
"They ended in Iceland by October. So Emma has been working and doing something, but on your own it is never the same.
"She may play some minutes, but she won't be a starter just yet as we will have to build up her fitness level and also her understanding of what our style is, and what is expected of her.
"The process to get used to the team and the people will be very short, however, as she already knows some of them, they played together in the young England teams. And she has been a teammate of Lucy Staniforth at Sunderland."
Tejedor added: "Emma looks like a very open personality, very intelligent.
"I was surprised in her first training session when she was calling almost every teammate by their name. I was like 'how did you learn everyone's name in half-an-hour?'. So her adaptability and capacity to get settled here will be, I think, very good."
The signing is pending International Clearance and Emma will wear the number 14 shirt.