Ulubat is the name of a freshwater lake, also called Apolyont Gölü in the vicinity of Bursa, Turkey. It is a large lake, covering an area of between 135 and 160 km², depending on the water level,but it is shallow, being only 3 m. deep at its most profound point. The lake contains eight islands, and one other that is sometimes an island and at ohter times a peninsula. The largest island is known as Halibey Island. In the southwest the lake is fed by the Mustafakemalpaşa River, which has formed a silty delta. Water leaves the lake by way of the Ulubat stream, flowing to the west, and reaches the Sea of Marmara via the Susurluk stream.

Most shores of the lake are covered in submerged plants, and it has the most extensive white water lily beds in Turkey. Ulubat Lake is one of the brooding areas for the endangered small cormorant (Phalacrocorax Pygmeus). The latest DHKD (Society for the Protection of Nature Turkey) survey (June 1998) found 823 pairs of Pygmy Cormorants, 105 pairs of Night Heron, 109 pairs of Squacco Heron, and 48 pairs of Spoonbill breeding on Ulubat.

The alternative name Apolyont comes from the ancient Greek name Apollonia,the name of an ancient city (Apollonia ad Rhyndacum) that was once situated on its banks, which had considerable importance since it was on the main trade routes. Modern residential areas by its shores are Mustafa Kemal Paşa (known to the Greeks as Kirmasti) and Karacabey(formerly known to the Greeks as Miletopolis).The area was famous for centuries for its silkworm cultivation, but this industry has died out, due to synthetic fabrics.The main industry these days is fishing.