Tomlin has long been linked with a move away from London Road, with Queens Park Rangers reportedly interested earlier in the transfer window.
However, he has now signed on loan at the Riverside Stadium and will agree a permanent contract in the summer.
Peterborough chairman Darragh MacAnthony said on his Twitter account: "Gutted to say Lee Tomlin has left on loan to permanent deal to @Boro in large transfer deal for all concerned. We wish him well!"
Tomlin joined the Posh in 2010.