The Nigeria international continued his superb scoring form for the Turkish giants as they booked their spot in the showpiece with a 5-2 aggregate win over Yeni Malatyaspor.
Following a goalless draw in the first-leg in Istanbul, within which Christian Luyindama saw red, Galatasaray bounced back with a comprehensive victory on the road.
Onyekuru played a key role in the turnaround at Malatya Arena, setting up the first before bagging a brace of his own.
Algeria international Sofiane Feghouli scored a goal and added an assist to take his overall tally for the season to 10 goals across all competitions.
Onyekuru, 21, who is on-loan from Everton, has scored a total of 14 goals.
The final will take place on the 16th of May.