Born in Florida, Aydınlık, 18, moved to Turkey at a young age, joining Fenerbahçe in 2015.
After starting out as a left-back, he has been switched into a forward/winger and is viewed as a star in the making in Turkey.
He has represented Turkey at under-19 level.
The signing of Aydinlik follows on from the club’s £5m signing of Aston Villa winger Jaden Philogene - around a week after he came to East Yorkshire to complete a medical.
The signing announcement had been delayed this week. Vila have inserted a buy-back clause into the deal and will have first refusal if Hull elect to sell the 21-year-old in the future. There is also an additional sell-on clause.
City have also been brought in Brighton left-back James Furlong on a three-year deal, with an option for a further year.
Outward wise, Oscar Estupinan - City's top-scorer last term - has left the Championship club to link up with ex-Tigers colleague Benjamin Tetteh at Ligue 1 outfit Metz.
Xavier Simons and Brandon Fleming has joined League One duo Fleetwood and Shrewsbury on loan respectively.