Turnbull made 27 appearances for the Tykes during two separate loan spells around a decade ago and the 29-year-old is back at Oakwell on a permanent basis after signing a two-year deal.
The 6ft 3in shot-stopper represented England at various youth levels while coming through the Middlesbrough academy and has also had stints with Darlington, Bradford, Crewe, Cardiff and Doncaster before spending four years with Chelsea.
"As soon I knew Barnsley were interested, I wanted to sign," said Turnbull. "I know Mark Crossley from my time at Middlesbrough as well. He was very complimentary of the club and now I'm looking forward to the new season."