Ball produced a neat finish nine minutes from time as the Rams were once again left to rue the lack of a cutting edge that has frustrated them on so many occasions this season.
Theo Robinson's 12th goal of the season in the first half had put the Rams on course for their 12th home win of the season and they had several chances to extend their lead. But they were caught on the break as Ball secured a point for Posh and it could have been even better for Darren Ferguson's side as George Boyd was denied by the post two minutes from time.
Rams boss Nigel Clough urged his side to finish the season on a high note ahead of the game and they were soon on the front foot.
Posh goalkeeper Paul Jones made a fine save to deny Steven Davies after the Rams' leading scorer was picked out by marauding full-back John Brayford eight minutes in.
Jones was called into action two minutes later, this time tipping Davies' well struck free-kick around the post.
But the Rams were in front after 20 minutes when supporters' player of the year Craig Bryson produced an inviting cross and Robinson got in front of his marker to apply the finish with the slightest of touches.
Derby then had a big shout for a penalty after Nathan Tyson raced into the box and appeared to be up-ended, but referee Roger East waved play on.
Academy graduate Will Hughes looked at home in his first start for the Rams and the midfielder picked out Bryson with a fine pass, but the former Kilmarnock man's cross fizzed across goal with Davies unable to finish it off.
Derby carried on where they left off after the break and Davies had the ball in the back of the net 50 minutes in but the assistant referee had his flag up for offside.
Tyson then raced away down the left and got to the byline before driving the ball across the face of the goal as the Rams pressed for a second goal.
Posh looked short of ideas going forward and Derby were bossing the midfield battle, with Bryson, Jeff Hendrick and Hughes running the show.
Posh almost caught the home side on the break with 15 minutes remaining, when Lee Tomlin's left-foot strike trickled just wide, and the Rams failed to heed the warning as Ferguson's side equalised nine minutes from time when Ball raced clear and curled a fine right-foot finish beyond Adam Legzdins and into the far corner of the net.
Boyd then came close to a winner for Posh when his strike came back off the post and Davies could have won it for the Rams but his header was tipped away by Jones right at the death.