As with all international travel, one should check TIMATIC for the precise language. For a US national traveling nonstop from the US to Turkey:-

Until 30 May 2021, passengers must have a negative COVID-19
PCR test taken at most 72 hours before departure from the
first embarkation point.

I have no idea whether that might vary for other permutations and thus you will need to double-check based on your specific situation. You should also continue to check as this is a requirement which could change on short notice.