There're mixed opinions about it but according to NOTAM (https://www.avianews.com/world/2020/...rpassengers/):NOTAM A6712/20 (Issued for LTAA LTBB) — COVID-19 / NOTAM REGARDING FLIGHTS TO TURKEY

1. STARTING FROM 29 DECEMBER 2020 21:00 UTC HOURS ALL PASSENGERS
THAT ARE 6 YEARS OLD AND OLDER WILL BE REQUIRED TO SUBMIT A NEGATIVE
SARS-COV-2 PCR TEST RESULT DONE WITHIN THE LAST 72-HOURS PRIOR TO
THEIR ENTRANCE TO TURKEY. PASSENGERS THAT DO NOT SHOW NEGATIVE
SARS-COV-2 PCR TEST RESULT WILL NOT BE ACCEPTED TO FLIGHT. THIS
REQUIREMENT IS APPLICABLE ALSO FOR TRANSIT AND TRANSFER PASSENGERS.

2. THESE MEASURES WILL BE IN FORCE UNTIL 01 MARCH 2021.

29 DEC 21:00 2020 UNTIL 28 FEB 21:00 2021 ESTIMATED. CREATED: 26 DEC 11:01 2020

However, people have been also reporting (for example on TK's Facebook) that according to information received from TK, transit passengers are excluded from test requirement.

It is hard to say definitely. We have to wait for official announcement, if such is published...