The forward had been strongly linked with a move to Posh with a deal now finally brokered, with the London Road outfit agreeing to pay a higher portion of his wages to facilitate his switch to East Anglia.
Mclean, reportedly the top earner at Valley Parade, has failed to make an impact with the Bantams after joining from Hull City.
He is now hoping to kick-start his career back at Posh, and if all goes well, a permanent move in January could transpire.
The Londoner hit 82 goals in a prolific spell at Posh between 2006-2010, helping them to back-to-back promotions to the Championship.
Bantams boss Phil Parkinson said: “It’s a good move and I am pleased for Aaron. It’s a good opportunity for him to go down there and get some games.
“Peterborough expressed an interest as soon as we made him available and that’s where he preferred to go.”
A couple of League Two sides, including Portsmouth, were also interested in taking Mclean, who has joined Posh for a ten-week period.
After the first month, Bradford will have a 24-hour recall clause.
Huddersfield Town first-team squad players Matt Crooks and Daniel Carr have returned to the John Smith’s Stadium after loan spells at Hartlepool United and Mansfield Town respectively.