The 24-year-old passed a medical on Tuesday afternoon following talks with the Magpies' boss as West Ham attempted to hijack a £5.5million deal.
Barton Signs!
Manchester City midfielder Barton headed across the Pennines after being given permission to talk to Newcastle, and arrived at the club's Darsley Park training ground along with Allardyce.
He left with Newcastle physio Derek Wright several hours later to start the medical as agent Willie McKay continued negotiations at St James' Park, but later made his way to the stadium to seal his move.