The 23-year-old was signed by former manager Gianfranco Zola in January on a three-and-a-half-year contract for a fee believed to be around £2.2m.
Frei made 13 appearances for the Championship side, 10 of them from the substitute's bench.
His only goal for the West Midlands club came in a 1-1 draw at Rotherham.
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Midfielder Kerim Frei has left Birmingham City to join Turkish side Istanbul Basaksehir on a four-year deal for an undisclosed fee.