The 32-year-old former Celtic, Dundee and Queen of the South keeper departed for Dingwall in 2015 but now returns to the Energy Check Stadium at Firhill.
Fox told the club website: “Having played with Thistle previously, it did make the decision a little easier. I had five good years here, and a title win. Once I knew a return was possible and learned about the plans they have for this season, it was something that really interested me.”
