The 29-year-old midfielder currently plays for NK Varazdin, a Croatian professional football club based in Varazdin.
Tractor has previously hired former Iran coach Dragan Skocic.
Postonjski started his playing career in HNK Gorica in 2017 and has also played in Inter Zapresic and NK Zagorec.
He will replace Iraqi midfielder Safa Hadi in the Tabriz-based football team.