The 27-year-old turned down a new Firhill contract earlier this month after announcing he wanted to “push for a higher level” as he chases a Scotland call-up.
But he has chosen Dingwall as he his next destination after quitting the Jags, despite the Staggies finishing one place below his old side in this season’s Premiership standings.
Fox, who was called up to Gordon Strachan’s Dark Blues squad for the November 2013 friendlies with the USA and Norway, has now penned a two-year deal with County.
McIntyre was on the lookout for a new stopper after releasing both Mark Brown and Antonio Reguero last week.
Fox’s capture follows the signing of midfielder Ian McShane from Queen of the South.
McIntyre said: “I’m delighted to add a goalkeeper of Scott’s calibre to the squad. He’s a player I’ve admired for a long time, very much a modern day keeper.
“He’s good with the back pass, kicks the ball well and has a safe pair of hands.”