The 28-year-old, who has scored seven times in 26 appearances for Posh this season, will bolster manager Steve Cotterill's striking options.
Cotterill told the club's official website said: "I'm delighted to have Tyrone on board. He offers us something different in the striking department.
"We'll have to work on his fitness because he hasn't played too many games recently. I'm sure he'll fit in well with the group because he's a good character."
Barnett joined Peterborough on a permanent basis in 2012 and had a short spell at Ipswich on loan during the last campaign, but an injury restricted him to just three appearances.