The 22-year-old becomes the Scottish Premiership club's fourth signing of the summer, after goalkeeper Jamie Sneddon, Niall Keown and Blair Spittal.
Turnbull's arrival came as captain Abdul Osman said Thistle needed more new recruits.
"I would say we need some new signings if we're going to compete for the top six again," said the midfielder.
Having graduated from Southampton's youth academy, Turnbull joined the Sky Blues in August on a three-year contract.
He made 46 appearances as Coventry were relegated to League Two.
During his time at Southampton, Turnbull spent two seasons on loan at Swindon Town, where he was named the club's player of the year in 2015.
Turnbull won a solitary England Under-20 cap and bolsters Thistle's defensive options after Liam Lindsay was sold to Barnsley earlier in the summer.
"We've obviously lost our centre-back who did very well for us last season, Liam, and the gaffer's working hard to bring in new recruitment," said Osman.
"But I'm sure the gaffer is doing his homework.
"We are a club on the up. Since I've been here, every year we've progressed.
"We've signed some good players and the recruitment's been good."
Osman thought that success this season would be to repeat that top-six finish.
"There's no reason why we can't do it," he said. "I'm quite confident.
"It was a great feeling, the fans loved it and the buzz around the place was amazing.
"It is a harder league this year, obviously with Hibs coming back up.
"But we're growing all the time and I think we can still do it.
"We just have to start the season off sharp. Last season, we started off slow - and the season before - and we can't afford to do that this year.
"Hopefully in the first 10 games, you're going to see a different Thistle."
The Glasgow side travel to Easter Road to face promoted Hibernian in their Premiership opener on Saturday.
"They are a big team in Scotland and it's going to be difficult, but it's a game we can go and win," added Osman.
Find all the latest football transfers on our dedicated page.
Partick Thistle have signed Coventry City defender Jordan Turnbull on a season-long loan deal.