Kasımpaşa suffered their first loss of the season following the 1-0 loss against Gençlerbirliği at the 19 Mayıs stadium.
The Istanbul minnows also conceded their first goal this season in the process.
On-loan Chelsea defender Kenneth Omeruo started for the Istanbul based side but was unable to help his side keep their third consecutive clean sheet.
The Nigerian international joined Kasımpaşa on-loan last month and the Istanbul based side have a buyout clause option.
The 21-year-old spent last season on loan at Middlesbrough who missed out on Premier League promotion.
GENÇLERBİRLİĞİ 1-0 KASIMPAŞA
Stadium: 19 Mayıs
Referee: Barış Şimşek
GENÇLERBİRLİĞİ: Hopf, Ahmet Oğuz, Ahmet Çalık, Atta, Uğur Çiftçi, Spelmann (min. 79 Berat Tosun), Skulason, Dimitriadis, Tomic (min. 58 Guido Koçer), İrfan Can Kahveci (min. 89 Doğa Kaya), El Kabir
Manager: Naci Şensoy
KASIMPAŞA: Isaksson, Veysel Sarı, Omeruo, Titi, Abdullah Durak, Donk, Aydın Yılmaz (min. 46 Eren Derdiyok), Pereira (min. 78 Sanharip Büyüksal), Valle, Adem Büyük (min. 78 Tunay Torun), Scarione
Manager: Rıza Çalımbay
Goal: min. 70 İrfan Can Kahveci (Gençlerbirliği)
Yellow Cards: min. 31 Ahmet Çalık, min. 81 Ahmet Oğuz, min. 85 Dimitriadis (Gençlerbirliği), min. 43 Adem Büyük, min. 51 Veysel Sarı, min. 83 Sanharip Büyüksal (Kasımpaşa)