Gemlik is a harbor town bordering the Sea of Marmara in Western Turkey, at approximately 29 kilometres from Bursa and not far from Istanbul. Gemlik was called Kios until 1922 when its Greek inhabitants (around 80% of the population) left Asia Minor because of the population exchange. In 2004, Gemlik had approximately 70,000 inhabitants. The harbour is one of the most important in Turkey. Somewhat east of Gemlik, a few remains of the ancient Greek town of Kios can be found.

It is also famous all over the world with its olives and oils. Marmara Birlik, which is a society of olive farmers and producers, has ranked as top 1 among the olive producers in the world.

The renowned Kurşunlu beaches is in Gemlik.

Giannis Papaioannou the Greek Rebetika singer and composer was born in Kios in 1913.

Gemlik's sister cities are Florence, Italy and Lauderhill, Florida, the United States.