Roma midfielder Nicolo Zaniolo was undergoing a Galatasaray medical in Istanbul on Tuesday night.
The Italy international flew out from Milan in the afternoon to complete the transfer. Roma have accepted an offer of 15m plus up to 7m in various add-ons.
The deal will also include a buyout clause of 35m.
Sportitalia says Zaniolo landed safely in Istanbul and was immediately taken to undergo the first part of the medical tests.
More tests will be performed in the morning, with the Turkish transfer deadline falling on Wednesday evening.