Biržai Castle is a castle in Biržai, Lithuania. Construction of the earth bastion-type castle started in 1586 by the order of Krzysztof Mikołaj "the Lightning" Radziwiłł. In 1575, preparing for this construction, a dam was built on the Agluona and Apaščia rivers at their confluence, and the artificial Lake Širvėna, covering about 40 km², was created. Major castle building works were finished in 1589.Since the second half of the 17th century, the castle has been the main seat of the Biržai-Dubingiai Radziwiłł family line, which was transferred here from the Dubingiai Castle. Biržai Castle served as a major defensive structure during the wars with Sweden.
The castle was reconstructed from ruins in the 1980s, in the Renaissance-Baroque style. The residential manor of the castle houses a library and a regional history museum "Sėla", founded in 1928.