Lake Koroneia (Greek: Λίμνη Κορώνεια) is a lake in the heart of the Thessaloniki regional unit in the Mygdonian basin in Greece. The lake is also known as Lake Agios Vasileios after the village Agios Vasileios and as Lake Langadas after the town Langadas. It is located about 14 kilometres east of Thessaloniki city centre. The motorway Egnatia Odos passes along the north side of the lake, and the old Greek National Road 2 passes along the south side. The lake is shared by the municipalities Langadas and Volvi.
A million years ago, Lake Koroneia along with Lake Volvi and all of the Mygdonian basin was a large lake. Since then, the area of the lake is declining and the distance from neighboring Lake Volvi is becoming larger. In the 1950s, Koroneia was among the lakes of Greece with the biggest fish production. In the 1970s, its area was 45 km² and had a depth of about 5 metres. Since then the lake has shrunk to about one third of its original area, and its depth has decreased to less than 1 metre. In the summer of 2009, it was possible to walk across the lake. In spite of being a wetland of international importance, as recognized by the Ramsar Convention, the lake suffers from pollution and intensive agriculture. The European Commission has referred Greece to the European Court of Justice for its failure to protect Lake Koroneia.