Širvėna Lake is a lake located in Biržai municipality, northern Lithuania. It is the oldest artificial lake in Lithuania. It was created in 1575 when the dam was built on rivers Agluona and Apaščia. The lake was built to protect the Radvila castle in Biržai. Now the city of Biržai, which was built around the castle, is located on the southern shore of Širvėna lake.