The 18-year-old attacker has opted to represent Turkey at senior international level despite having previously appeared for Switzerland's Under-19 and Under-21 sides
Born in Austria, the 18-year-old was raised in Switzerland and has gone on to represent his adopted nation at Under-19 and Under 21 level.
However, due to Frei's father being Turkish, the youngster also had the option to represent Turkey in addition to Austria and Switzerland, and in the end has been convinced to appear for the country of his father's birth.
Turkey national team coach Abdullah Avci played a key role in persuading Frei to switch national allegiances and he will now have the young prospect at his disposal in the team's friendly against Austria on August 15.
It will be a return to the country of his birth for Frei, with the match set to take place at Vienna's Ernst Happel stadium.
Once said to be courted by Chelsea, Frei is now entering his second season at Premier League side Fulham, for whom he made 16 appearances last term.