The 29-year-old has signed a two-year deal with the Reds, making him a Barnsley player once again after featuring at Oakwell 27 times across two loan spells during the 2003/4 and 2004/5 seasons.
Having started his career with Middlesbrough, he spent seven years as a professional at the Riverside whilst also representing England at Under 16, 17, 18 and 19 levels.
After his temporary spell with the Reds, he featured out on loan with Crewe and Cardiff before eventually heading to the capital with Chelsea.
In his four years at Stamford Bridge, Turnbull picked up a Champions League and FA Cup Winners medal.
"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."