The 27-year-old midfielder has also previously played for Livingston and Airdrieonians and now makes the move to Firhill.
Jags boss Ian McCall told the club website: "I’ve worked with Ross for a long time - he’s given five years of terrific service at Ayr, where he was my captain, and his level of consistency throughout that time was excellent. He’s a fine footballer but it’s also Ross as a person that will improve what we have at the club – he is a winner."
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