The Catalan giants will pay an initial 2 million euros (2.3 million dollars) for the 17-year-old.
Demir will remain with Super Lig side Kayserispor for the remainder of 2021/2022 before linking up with Barcelona B from the start of next season.
He has already featured 31 times at senior level and made history in November 2019.
That was when he became the youngest scorer in the Turkish top-flight at 15 years, nine months and 25 days old.
The youngster, who previously had a trial at Barca’s La Masia academy in 2017, will become the third Turkish player to sign for the Catalans after Rustu Recber and Arda Turan.
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