Tommy Rowe's header denied high-flying Middlesbrough the chance to draw level on points with leaders Southampton.
Scott McDonald gave Boro the lead when he headed in Barry Robson's corner, before the goalscorer twice hit the bar with thunderous efforts.
But winger Rowe salvaged a point for Posh when he headed into the roof of the net from a corner.
It was the first time Tony Mowbray's Boro had dropped points in December after four successive wins.
However, defeat for West Ham meant that Boro moved into the second automatic promotion spot, thanks to that point.
McDonald looked lively early in the match, firing in two shots that crept over the bar, before he narrowly failed to get a touch on a cross from Justin Hoyte.
Posh were better after the break, with George Boyd forcing Middlesbrough keeper Jason Steele to palm away his strike.
Scott Wootton and Paul Taylor also went close with fierce shots from distance.