The Turkey international would be paid 2 million euros (£1.8 million) plus a bonus for this season and 4 million euros next season, the club chairman Goksel Gumusdag announced.
"I am excited to be back to doing what I love most: playing football. I missed football so much," Turan said after not making a single appearance for Barcelona this season.
One point clear of city rivals Galatsaray, who Turan played for from 2005-11, after 17 games, the midfielder's arrival is expected to help push the club to a maiden Super Lig title.
Turan will find himself in a dressing room of a an ambitious club with a growing pedigree as former Manchester City pair Emmanuel Adebayor and Gael Clichy are among the most high-profile members of the current squad.
"I think we are one of the most important candidates for the championship this season. We will play each and every game like we are playing the most important final of our lives," he said.
The 30-year-old joined Barcelona from Atletico Madrid for 34 million euros plus add-ons in July 2015, winning six trophies but only making 55 appearances and scoring 15 goals at the Camp Nou.
Turan has two-and-a-half years left on his Barca contract but will see it out in Istanbul. However, Barcelona can still sell the player during that period if they accept an offer for him.
Spanish newspaper Sport last week estimated it cost Barca one million euros for each appearance made by Turan, taking into account an annual salary reported to be 8 million euros a year.
Turkey's fourth-most capped player with 100 appearances, Turan will also be hoping to return to his best form for the national side after reversing a decision to retire from international football in August.
Turan quit in June but changed his mind a month later after positive talks with new head coach Mircea Lucescu.
(Editing by John O'Brien)
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