The 19-year-old heads to Firhill on a season-long agreement designed to give him regular first-team experience after previous loans at Spartans, Stirling Albion and Albion Rovers.
Stone is considered one of the most prodigious talents to emerge from the Riccarton youth academy and is seen as a future first-choice goalkeeper at Hearts.
He impressed in recent pre-season friendly matches but needs to play more competitive matches in order to develop. With newly-appointed captain Craig Gordon firmly established as Hearts’ No.1, Stone has taken the chance to move to Glasgow for season 2021/22.
Thistle won promotion to the Scottish Championship last season and manager Ian McCall wants his team to compete in the second tier. He has brought in Stone to compete with Jamie Sneddon for the goalkeeping position.
“We’re extremely happy to get Harry’s loan over the line,” McCall told his club’s website. “It’s a deal we’ve been working on for a while but it’s good to get it all finalised. He’s already joined up with us for training and he looked very sharp. He’ll be a real asset for us this season.
“Harry comes very highly rated and similar to last season we have really good competition for the goalkeeper position. It’s now up to Jamie and Harry to earn the right to start for the team.”
Stone recently explained his future plans in an exclusive Evening News interview.