Osmaneli is a town and district of Bilecik Province in the Marmara region of Turkey with a population of 13,816 (as of 2010). The mayor is Mustafa Selahettin Çetintaş (AKP).
Situated on a hillside at a point where the Karasu, flowing down from Kandilli Dağ, enters the Sakarya, this small town was known in antiquity as Leukai and more recently as Lefke. South of the town is a narrow and very beautiful river gorge with rocky walls up to 100 m/328 ft high, contributing to some spectacular scenery.
In the west part of Osmaneli are the well-preserved ruins of a large Rum (Anatolian Greek) Orthodox church of 1890's not far from it, a building that was used as a silk-spinning mill, and also the owner's house. There are several Ottoman style homes available and watermelon festival each year being celebrated, which is also worth seeing.