The 24-year-old midfielder arrives in the capital from relegated FC Schalke 04. The two clubs came to an agreement on the deal on Friday but have agreed not to disclose details of the transfer.
The four-time Germany international came through the academy at 1.FSV Mainz 05 and made the leap to the professional game with the Zerofivers. Serdar then made the move to Gelsenkirchen in 2018 and joins Hertha BSC after three years at Schalke.