The 24-year old Tomic won four successive league titles with Partizan, scoring 31 goals in 159 games for the club in all competitions. He has made five international appearances and scored one goal.
He will join compatriots Dusko Tosic, former Blackburn Rovers midfielder Radosav Petrovic and striker Dejan Lekic at Genclerbirligi, who are 11th in the 18-team Turkish first division.
Partizan, who are chasing a record sixth consecutive league title, could offload more players who are surplus to requirements in the next few days in a bid to cut costs.
Striker Stefan Scepovic and defender Sreten Sretenovic, signed by Partizan during the close season, are on the verge of joining Korean top flight side Busan while Sierra Leone midfieldfer Mohamed Kamara is also set to leave the club.
"There is a 99 percent chance that the former two will sign for Busan and the board will meet on Friday to hammer out the details and do the paperwork so that they can leave," Partizan's managing director Ljubisa Tumbakovic said.
Partizan are six points clear of city rivals Red Star at the mid-season break in Serbia, where the 16-team first division resumes on February 27.