Turnbull - who scored a superb hat-trick on the opening day of the new Guinness PRO12 season against Zebre - has been a huge success since moving to Cardiff Blues from the Scarlets in 2013.
The powerful forward made 29 appearances in his debut season at BT Sport Cardiff Arms Park, scoring three tries, as he added his talent and vast experience to the pack at Blues.
Turnbull started his regional career with Scarlets in 2007, having previously played for Carmarthen Quins and Llanelli, and went on to make a total of 119 competitive appearances.
The 27-year-old has also impressed on the international stage having represented Wales at Under-18, Under-19 and Under-20 levels and participated in two IRB Under-19 World Championships.
The Haverfordwest-born flanker was named in the Wales senior squad for the first time in 2011 and made his debut as a replacement in the 6 Nations win over Scotland at Murrayfield. He was also selected for the 2012 tour to Australia and 2014 tour to South Africa.
Head Coach Danny Wilson said: "We're thrilled that Josh has signed a contract extension He is a player I know extremely well, having worked together at Scarlets, and I'm delighted that he is part of the set-up at Cardiff Blues.
"He is a real leader, someone who puts his body on the line and is a fantastic professional. He displays all the values that we are looking for in players who represent Cardiff Blues.
"Josh has repeatedly proven his quality at both regional and international level. He remains a very ambitious player and the fact he has signed a new contract is testament to what we are striving to achieve here.
"The news that both Josh Turnbull and Josh Navidi have signed new contracts in recent weeks is a big boost as we start the new season and look to build a squad to bring us long-term success."
Turnbull said: "I'm really pleased to sign for another two years, on top of my existing deal, and that I've been able to show my commitment.
"I can now look forward to the season ahead and the next three years at Cardiff Blues, especially with Danny Wilson coming in because he was a big factor in me staying.
"We're looking to build something special over the coming years and we showed in our first league game with Zebre the potential we have here at Cardiff Blues."
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