Foster, from Ontario, Canada, joins the Reds after finishing her career in the American college system with West Virginia University.
The 21-year-old enjoyed a successful four-year career with the Mountaineers, playing 84 games and recording 39 clean sheets.
She was named on the MAC Hermann Trophy long list for the most outstanding college player of the year in two successive years.
A Canada U20 international, she will wear the No.13 jersey with the Reds.
LFC Women manager Vicky Jepson said: “Rylee is a good signing for us.